This study was conducted to determine, on a prospective basis, the incidence of acute exertional rhabdomyolysis (AER) among recruits at the Marine Corps Recruit Depot, San Diego, Calif. Blood samples were taken from each of 337 volunteer recruits on each of their first six days of regularly scheduled training. Serum myoglobin, serum creatine phosphokinase, lactic dehydrogenase, and serum glutamic oxaloacetic transaminase values were used as indicators of muscle injury. Substantial elevations of serum enzyme activity were observed throughout the study population. Of the study population, 39.2% had serum myoglobin levels that ranged from 0.37 mug/ml to 21.9 mug/ml during the study interval. Six subjects had serum myoglobin levels consistent with those reported in clinical cases of AER. It is concluded that, in a recruit population, large numbers of men may have myoglobinemia but not be seen initially as clinical cases.