In a prospective study of stress injuries of the lower extremities of U.S. Marine recruits, we derived a frequency distribution of stress fractures. The most frequently fractured bone was the tibia (73%), while the single most common site was the posterior calcaneal tuberosity (21%). The natural history of stress fractures by scintigraphy and radiography has been outlined, showing the evolutionary changes on either study as a universal progression independent of injury site or type of stress. An identical spectrum of changes should be present within any group undergoing intense new exercise. The frequency distribution of stress fractures should be a function of differing forms and intensities of exercise, therefore, our figures should not be applied to other groups. We used the presence of a scintigraphic abnormality at a symptomatic site as the criterion for diagnosis of stress fracture. Since the distribution of skeletal radiotracer uptake is directly dependent on local metabolic activity, it is expected that a focal alteration in bone metabolism will result in a scintigram approaching 100% sensitivity for the abnormality (9). In the proper clinical setting, the specificity should approximate this figure; however, a focal, nonstress-related bone abnormality which has not manifested any radiographic change, such as early osteomyelitis, could result in a false-positive examination. Specificity cannot, therefore, be accurately determined without an actual determination of the pathologic changes within the bone, necessarily involving biopsy. In summary, we believe that we have established bone scintigraphy as an early and accurate means for the detection of lower extremity stress fractures, even in the absence of radiographic findings (6). We feel that a focally abnormal scintigram, in the proper clinical setting, establishes the diagnosis of stress fracture, with radiography to be performed at the time of initial work-up only to rule out a non-stress injury (such as complete fracture, fibrous dysplasia, osteomyelitis, primary bone tumor).