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Seborrheic dermatitis (SD) is a chronic relapsing dermatosis affecting all ages, characterized by pruritic erythematous plaques with greasy scale on sebum-rich areas; scalp-limited disease presents as dandruff. Management typically includes antifungal agents, intermittent low-potency topical corticosteroids, steroid-sparing topical calcineurin inhibitors or phosphodiesterase-4 (PDE4) inhibitors, and keratolytics to reduce scale. SD impairs quality of life through itch, visible scale, and social stigma. Contemporary United States (US) epidemiology of SD is seldom summarized for clinicians beyond single-center or payer cohorts. We analyzed US national and state-level SD burden from 2010–2021 using the Global Burden of Disease (GBD) 2021 study methodology, which employs Bayesian meta-regression modeling (DisMod-MR 2.1) to synthesize US insurance claims databases and national health surveys. Case definition followed GBD mapping for SD (International Classification of Diseases [ICD-10] L21–L21.9; ICD-9690–690.9) [1]. In 2021, the US reported 6.21 million new SD cases (95% uncertainty interval [UI]: 5.77–6.61 million), a 4.6% increase from 2010. Despite rising crude case numbers, the age-standardized incidence rate (ASIR) remained stable at 1913.1 per 100,000 (95% UI: 1771.6–2048.7). Prevalent cases reached 1.29 million (95% UI: 1.22–1.37 million), increasing 16.0% since 2010, while the age-standardized prevalence rate (ASPR) remained stable at 319.2 per 100,000 (95% UI: 301.8–338.2). While GBD provides a robust tool for analyzing trends, its estimated all-age prevalence for 2021 [0.41% (95% UI, 0.39–0.43)] is lower than estimates from observational studies (~1%–5% in adults), likely because GBD harmonizes multiple data sources and is anchored in diagnosed, medically encountered cases, and may therefore appear lower than community surveys that also capture undiagnosed or self-treated disease [2]. Mendeley supplementary materials provide full state, age, and sex stratifications, and methods. Notable geographic variations emerged across states (Table 1, Figure 1). The District of Columbia (DC) demonstrated the highest ASIR (2386.5 per 100,000), followed by Hawaii and Florida, while New Mexico showed the lowest (1554.2 per 100,000), representing a 53.5% relative difference in incidence rates. Similar patterns occurred for prevalence rates, with DC showing 114% higher ASPR than New Mexico. DC's elevated rates may reflect both environmental factors and enhanced healthcare access, leading to improved diagnosis capture, highlighting the complex interplay between true prevalence and detection bias. Incidence peaked during middle adulthood (30–50 years), whereas prevalence and burden peaked in older adults (approx. 75–84 years). Women had higher age-standardized incidence than men (2350.2 vs. 1910.5 per 100,000; 23% difference). Prevalence patterns largely mirrored incidence, though with minor variations in state rankings for the lowest-burden states (Table 1). Stable age-standardized rates alongside rising case counts suggest no material change in national risk despite population growth. The variation across states in our estimates may suggest differences in environmental exposures or healthcare access, but it remains hypothesis-generating. Higher burden in warm, humid regions (Florida, Hawaii, DC) vs. continental climates (Wyoming, Maine) is consistent with hypothesized climatic influences on Malassezia growth and sebum production; winter-worsening reported in temperate settings may be less pronounced in humid subtropical regions [3, 4]. Diet may also play a role: one study linked higher fruit intake with lower odds of SD and a Western dietary pattern with higher odds in women [5]. Clinically, awareness may be particularly important in mid-adult women and in patients with comorbid human immunodeficiency virus (HIV) or Parkinson's disease, where SD prevalence is elevated. Study limitations include potential underestimation of mild cases, variations in healthcare-seeking behavior, and the absence of climate covariates in current GBD models for SD. However, the inter-state variations warrant investigation into specific environmental determinants (humidity, temperature, ultraviolet [UV] exposure, air quality) and their interaction with host factors. Future research should explore whether patients relocating between high- and low-burden states experience changes in disease severity, which could provide insights into modifiable risk factors. This analysis received no funding. We used GBD 2021 estimates; the GBD enterprise is funded by the Bill & Melinda Gates Foundation, which had no role in this work. A.G. is an unpaid official Senior Collaborator for IHME/GBD. He serves as an Associate Editor for the Journal of Investigative Dermatology and the International Journal of Dermatology, a Physician Editor for VisualDx, a member of the Board of Directors for the Biology of Skin Foundation, and a Medical Director at AbbVie. D.G.C. has served as a consultant, speaker, and/or investigator for AbbVie, Alumis, Arcutis, Boehringer Ingelheim, Bristol Myers Squibb, Castle, Dermavant/Organon, DermTech, Eli Lilly, Galderma, Johnson and Johnson, Journey, KOWA, Pfizer, Regeneron, and Sanofi, UCB. C.G.B. has served as an investigator for AbbVie, Almirall, Apogee, Daiichi Sankyo, LEO Pharma, Ortho Dermatologics, Sun Pharma, Takeda, Timber, and Palvella; a consultant for AbbVie, Almirall, Alumis, Amgen, Apogee, Arcutis, Botanix, Connect BioPharma, Dermavant, Eli Lilly, EPI Health/Novan, Galderma, Incyte, LEO Pharma, Novartis, Ortho Dermatologics, Pfizer, Regeneron, Sanofi, Sun Pharma, Takeda, Teladoc, Triveni, and UCB. L.S., H.A., and M.G. declare no information. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.