Cardiac ankyrin-repeated protein (CARP) has been shown to associate with a transcription factor, YB-1, that may activate expression of the ventricular myosin light chain-2 gene during cardiogenesis. CARP is induced in the adult hypertrophic heart subjected to pressure overload, suggesting that CARP may play important functional roles in both embryonic and adult hearts. Although CARP expression was initially believed to be restricted to the heart, we found recently that CARP is induced strongly in human fetal skeletal muscle and in experimentally denervated skeletal muscle, leading us to speculate that CARP may also play important roles in skeletal muscle. In the present study, we found that in rats initially damaged by a single injection of bupivacaine, CARP expression was induced strongly in regenerating muscles with a peak 3 days after the injection, followed by down-regulation to undetectable levels after 28 days. Although CARP was coexpressed with embryonic myosin heavy chain (MHC) in regenerating myofibers, CARP expression persisted even after down-regulation of embryonic MHC expression, whereas it began to decrease before the onset of slow or fast MHC expression, suggesting that CARP is expressed at a specific differentiation stage during muscle regeneration. We analyzed the expression of CARP in muscle biopsy specimens from 14 patients with muscular dystrophy (MD) and detected high expression of CARP in 13 of the 14 cases. CARP-positive myofibers were detected more often in congenital muscular dystrophy (CMD) than in Duchenne muscular dystrophy (DMD). We found that CARP was expressed exclusively, and at a high level, in small regenerating myofibers that express embryonic MHC in DMD, which suggested that CARP could be used as a marker of muscle regeneration in DMD. On the other hand, in CMD, expression of CARP was not limited to regenerating fibers, being detectable in myofibers expressing embryonic MHC and those expressing mature-type MHC. These findings suggest that the differentiation stage of CARP-positive myofibers in DMD and CMD may differ.