A series of 142 stress fractures caused by sporting activities and physical exercise is presented. 121 fractures occurred in athletes and 21 in non-competitive sportsmen. Distance runners presented with 68 fractures, skiers 12, sprinters 10, orienteering runners 9, vaulters 3, and football-players 3 fractures. Athletes engaged in other events had fewer stress fractures. 76 fractures occurred in the tibia, 26 in the metatarsal bones, 20 in the fibula, 5 in the femoral neck, 4 in the femoral shaft, and 2 in the metacapal bones, lower pubic arch and sesamoid bones of the first MTP-joint. There was one fracture of each of the following: the humeral shaft, the ulna, the vertebral arch of L 5, the tarsal navicular and the proximal phalanx of the fifth toe. The treatment was generally a pause in training for 4-6 weeks, on the average. Running caused most of the stress fractures; the rest followed jumping exercises. The athletes mostly developed stress fractures during a period of alteration from one training session to another or during the preparation period close to the competition season. Joggers usually developped stress fractures 2-4 months after the beginning of regular training.