Members of the CELF family of RNA binding proteins have been implicated in alternative splicing regulation in developing heart. Transgenic mice that express a nuclear dominant-negative CELF protein specifically in the heart (MHC-CELFDelta) develop cardiac hypertrophy and dilated cardiomyopathy with defects in alternative splicing beginning as early as 3 weeks after birth. MHC-CELFDelta mice exhibit extensive cardiac fibrosis, severe cardiac dysfunction, and premature death. Interestingly, the penetrance of the phenotype is greater in females than in males despite similar levels of dominant-negative expression, suggesting that there is sex-specific modulation of splicing activity. The cardiac defects in MHC-CELFdelta mice are directly attributable to reduced levels of CELF activity, as crossing these mice with mice overexpressing CUG-BP1, a wild-type CELF protein, rescues defects in alternative splicing, the severity and incidence of cardiac hypertrophy, and survival. We conclude that CELF protein activity is required for normal alternative splicing in the heart in vivo and that normal CELF-mediated alternative splicing regulation is in turn required for normal cardiac function.