Eight young men undergoing heavy repetitive exertion manifested a syndrome consisting of severe myalgia, myoglobinuria, elevated serum levels of enzymes of muscle cell origin, and impaired muscle function. Stress testing after recovery did not reproduce this syndrome. Subclinical rhabdomyolysis was detected in a prospectively selected group of 38 young naval officer candidates. Rarity of overt rhabdomyolysis in a large homogeneous population of men exposed to similar exercise stress conditions in preflight training, and negatively skewed distribution of serum enzyme values in a sample of this population suggest that rhabdomyolysis causing impaired muscle function occurs after prolonged calisthenics in certain predisposed individuals rather than in all poorly conditioned men. Prior physical conditioning lessened the probability of this occurrence, supporting the recommendation for a graduated physical training program.