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The current view that death of dystrophin-deficient muscle fibers is a necrotic process relies primarily upon the histological appearance of the tissue after the degenerative process is well advanced. Here, we tested this view by examining the possibility that apoptosis is a component of dystrophin-deficient muscle cell death. Three assays for apoptosis were employed in analyzing prenecrotic, peak necrotic and regenerated hindlimb muscle of mdx mice: (1) terminal deoxynucleotidyl transferase (TdT) mediated end-labeling of DNA in nuclei in tissue sections; (2) assays for DNA ladders; and (3) electron microscopic assays for the presence of organelles undergoing structural changes characteristic of apoptosis. At all ages sampled, mdx muscle contained apoptotic nuclei, according to TdT-mediated dUTP labeling of tissue sections. Nuclei in regenerated mdx muscle fibers did not display apoptosis. dUTP-labeled nuclei in control C57 muscles were rare or absent at all ages sampled. DNA from 4-week-old mdx mice was found to be cleaved into fragments indicative of preferential cleavage at internucleosomal sites. Electron microscopic analysis showed that organelle structural changes indicating apoptosis appear before pathological changes diagnostic of necrosis. For example, condensed mitochondria, fragmented sarcoplasmic reticulum and nuclei with chromatin condensations resembling apoptosis appear in fibers that otherwise possess normal morphology. Together, the findings show that apoptosis precedes any detectable necrotic change in mdx muscle, and that apoptotic events continue into the stage of dystrophic pathology that is currently viewed as necrosis. Thus, apoptosis characterizes the onset of pathology in dystrophin-deficient muscle which is followed secondarily by necrotic processes.