Twenty-one marine recruits in three training programs at the Marine Corps Recruit Depot, Parris Island, SC, were prospectively studied for one week for the presence of myoglobinemia and development of clinical rhabdomyolysis. Myoglobinemia was found in two thirds of the recruits at least once during the study period despite the absence of clinical symptoms. Serum myoglobin value ranged from 0.30 to 3.60μg/ml and correlated poorly with serum levels of creatine phosphokinase, glutamic oxaloacetic transaminase, and lactic dehydrogenase. With the exception of one recruit believed to have subclinical exertional rhabdomyolysis, those recruits in apparently poorer physical condition had higher average daily serum myoglobin and enzyme levels. Eleven marine recruits who were hospitalized for exertional rhabdomyolysis are compared to the asymptomatic recruits. Serum myoglobin and enzyme concentrations were 10 to 100 times greater in the hospitalized men than in asymptomatic recruits.