A case is presented of a 19-year-old white female, detected while we were investigating the range of serum creatine kinase levels in “normal” controls. She presented elevated serum creatine kinase, myopathic electromyogram, and electron microscopic evidence of dystrophic changes in muscle tissue, but no clinically detectable muscular weakness. The significance of these findings is discussed. We felt that she was probably a sporadic carrier of the gene for Duchenne's muscular dystrophy and, because there was no family history of neuromuscular disease, that this was the result of a spontaneous mutation.