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Background Depression is an important public-health problem, and one of the leading causes of disease burden worldwide. Depression is often comorbid with other chronic diseases and can worsen their associated health outcomes. Few studies have explored the effect of depression, alone or as a comorbidity, on overall health status. Methods The WHO World Health Survey (WHS) studied adults aged 18 years and older to obtain data for health, health-related outcomes, and their determinants. Prevalence of depression in respondents based on ICD-10 criteria was estimated. Prevalence values for four chronic physical diseases-angina, arthritis, asthma, and diabetes-were also estimated using algorithms derived via a Diagnostic Item Probability Study. Mean health scores were constructed using factor analysis and compared across different disease states and demographic variables. The relation of these disease states to mean health scores was determined through regression modelling. Findings Observations were available for 245404 participants from 60 countries in all regions of the world. Overall, 1-year prevalence for ICD-10 depressive episode alone was 3-2% (95% CI 3-0-3-5); for angina 4-5% (4-3-4-8); for arthritis 4-1% (3·8-4·3); for asthma 3·3% (2·9-3·6); and for diabetes 2-0% (1·8-2·2). An average of between 9·3% and 23·0% of participants with one or more chronic physical disease had comorbid depression. This result was significantly higher than the likelihood of having depression in the absence of a chronic physical disease (p<0·0001). After adjustment for socioeconomic factors and health conditions, depression had the largest effect on worsening mean health scores compared with the other chronic conditions. Consistently across countries and different demographic characteristics, respondents with depression comorbid with one or more chronic diseases had the worst health scores of all the disease states. Interpretation Depression produces the greatest decrement in health compared with the chronic diseases angina, arthritis, asthma, and diabetes. The comorbid state of depression incrementally worsens health compared with depression alone, with any of the chronic diseases alone, and with any combination of chronic diseases without depression. These results indicate the urgency of addressing depression as a public-health priority to reduce disease burden and disability, and to improve the overall health of populations.