Duchenne muscular dystrophy leads to cardiomyopathy. The objective of this study was to estimate the association of body mass index with cardiomyopathy onset. Cardiomyopathy was defined as left ventricular ejection fraction <55% or left ventricular fractional shortening <28%. Overall, 48% met the criteria for cardiomyopathy. We were unable to demonstrate an association between body mass index Z score and age of cardiomyopathy onset (hazard ratio 0.79, 95% confidence interval 0.57-1.11, P = .17) after adjusting for covariates. Duration of corticosteroid use (P = .01), but not loss of ambulatory ability (P = .47), was associated with age of cardiomyopathy onset. We were unable to detect a significant difference in median body mass index Z scores in corticosteroid-treated boys compared with corticosteroid-naïve boys (1.11, 95% confidence interval 0.25-1.95, vs 1.05, 95% confidence interval 0.01-1.86, P = .69). No association was detected between the body mass index Z scores of Duchenne muscular dystrophy subjects and age of cardiomyopathy onset.