The occurrence of stress fractures was studied in 120 recruits after a progressive basic training period of ten weeks. The diagnosis made by bone scintigraphy, was confirmed by radiographs. The occurrence of stress fractures varied, depending on the amount of exertion required by the endurance training. Stress fractures were most frequent among sample C, the parachutists , 63%. The frequency of stress fractures in normal infantrymen of sample B was 35%. In sample A, light infantrymen , stress fractures occurred in 15% of the men. The location of the bone lesions was unexpectedly high in the upper legs.