An epidemiologic study of idiopathic dilated cardiomyopathy was carried out in order to identify possible risk factors for this often fatal cause of heart failure in young adults. Possible associations with black race and other genetic and environmental factors were examined by comparing newly diagnosed cases ascertained from four Baltimore hospitals (n = 95) with neighborhood controls (n = 95), matched on sex and 5-year age intervals. Matched and unmatched relative odds and conditional logistic regression coefficients were obtained to describe the associations. Statistically significant, independent associations were observed between idiopathic dilated cardiomyopathy and black race, low annual income, and history of asthma (p less than 0.05). The black predominance (relative odds = 2.7, 95% confidence interval 2.0-3.4) was not explained by income, alcohol consumption, cigarette usage, body mass index, hypertension, or asthma. A possible interactive effect was observed between black race and history of asthma and other atopic diseases. Thus, blacks, especially those with a history of hypersensitivity, may represent a high-risk subgroup in need of preventive care or early intervention.