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Abstract Introduction: Paroxysmal cold hemoglobinuria (PCH) is a rare cause of autoimmune hemolytic anemia (AIHA), most frequently observed in children following viral infections. It is mediated by Donath–Landsteiner (DL) antibodies, which induce intravascular hemolysis upon cold exposure and rewarming. The occurrence of PCH in adults is uncommon, and its co existence with monoclonal B-cell lymphocytosis (MBL), a premalignant clonal B-cell disorder is extremely rare, with few documented cases. This report highlights a unique diagnostic intersection and expands the differential for chronic anemia with hemolytic indices in elderly patients. Case Description: An 86-year-old male with a history of chronic anemia, atrial fibrillation, and transient ischemic attack presented for evaluation of fatigue and longstanding anemia. Labs showed hemoglobin 8.9–12.0 g/dL over several years, persistently undetectable haptoglobin, elevated indirect bilirubin, and reticulocytosis. Iron, B12, and folate were normal. Peripheral smear showed normocytic anemia without schistocytes or burr cells. Initial direct antiglobulin test (DAT) was negative; however, a super DAT was positive for cold agglutinins (titer 1:64), and Donath–Landsteiner antibody testing confirmed the diagnosis of PCH. Bone marrow biopsy revealed a small clonal CD5+, kappa-restricted B-cell population consistent with MBL. The patient was treated with weekly rituximab infusions for four weeks. No transfusions, corticosteroids, or IVIG were required. He remained clinically stable during and after treatment. Discussion: This case underscores the diagnostic complexity and rarity of PCH in elderly patients and its unusual co-presentation with MBL. While PCH is more often seen in children and usually post-viral, adult cases require a high index of suspicion due to nonspecific symptoms and often-negative DAT results. MBL, although often asymptomatic, may contribute to immune dysregulation and has been rarely associated with AIHA. The pathophysiology of PCH involves the DL antibody, an IgG autoantibody that binds RBCs at low temperatures and activates complement upon warming, leading to hemolysis. Early recognition of this biphasic mechanism is key. While rituximab was used effectively in our case, cold avoidance and treatment of underlying infections remain the cornerstone of therapy in typical cases. The patient's favorable response without escalation of therapy highlights the potential for targeted management in such rare presentations. This report contributes to the limited literature on PCH–MBL overlap and supports thorough hematologic workup in elderly patients with unexplained hemolysis.