Aged mdx mice represent an important model for studying Duchenne cardiomyopathy. Herein we compared the cardiac phenotypes of 22-month-old male and female mdx mice. Surprisingly, only females displayed the characteristic cardiac dilation on pressure-volume loop analysis. Female mdx mice also exhibited lower contractility, larger Q waves, and higher ratios of heart weight to body weight. Our results reveal significant gender disparity in mdx cardiac function. Gender should be considered when using the mdx model for the study of Duchenne cardiomyopathy.