Serum enzyme studies were made on a large, well-defined group of patients with neuromuscular diseases. Control values for creatine phosphokinase, aldolase, and lactic dehydrogenase were significantly higher in males. (P<.05,.005, and.01, respectively). A rapid drop in creatine phosphokinase was observed after age 10 in patients with Duchenne dystrophy, and after age 20 in carriers with elevated enzyme levels. A significant negative age-enzyme correlation for creatine phosphokinase levels in carriers was observed (r = —.48, P<.01). This emphasizes the importance of early carrier detection studies in the suspect female patient. Elevations in the Becker and Duchenne forms of X-linked dystrophy were found to be similar in frequency and degree, even though the rapidity of cellular breakdown is quite different. Of all the disorders studied, only spinal muscular atrophy demonstrated a positive correlation for creatine phosphokinase disease duration.