Ninety-four stress fractures were detected in 64 soldiers by scintigraphy during the first 11 weeks of basic training. Unlike past military studies that showed large numbers of stress fractures of the feet, only 2% were in the metatarsals and none were in the calcaneus. Most fractures were in the tibia (71%) and femoral shaft (25%). Contrary to previous reports in the literature, the fractures occurred later in training, with 67% occurring during the fifth to eighth weeks of basic training and 12 percent during the ninth to eleventh weeks. This unusual distribution and onset has possible explanations in (1) new training concepts to fit modern warfare, with more emphasis on runs and marches; (2) new apparel and equipment carried by the soldiers; (3) changes in prearmy lifestyle; and (4) the extensive use of bone scan for diagnosis.