Duchenne muscular dystrophy and Becker muscular dystrophy are X-linked disorders that result from a mutation in the dystrophin gene that reduces the production or effectiveness of the protein dystrophin. These disorders are clinically characterized by progressive muscle degeneration. Manifesting female carriers are generally not identified as such until after puberty, when symptoms such as muscle weakness may arise. This clinical report describes a female manifesting carrier who started to show deterioration of left ventricular systolic function, but no marked skeletal muscle weakness, at the age of 10 years. The patient's cardiac function improved significantly after dual drug therapy with an ACE inhibitor (enalapril) and a beta-blocker (carvedilol). Our case adds to the existing evidence that left ventricular myocardial dysfunction may occur during childhood in female carriers of dystrophinopathies. © 2011 Wiley Periodicals, Inc.