Exercise-associated amenorrhea is the cessation of menses in a woman following onset of training or an increase in training intensity. Its physiologic basis is characterized by consistently low levels of gonadotropin and ovarian hormones, but the underlying cause of this phenomenon is unknown. Although osteopenia has been described in amenorrheic women athletes, it has been primarily a laboratory diagnosis. Several recent studies have described a significantly lower bone mineral density (BMD) in the lumbar spine of amenorrheic athletes. Marcus et al. also reported an increased number of metatarsal and tibial stress fractures in a group of amenorrheic women. We report here the first case of a nontraumatic femur fracture in an amenorrheic athlete. A 32-yr-old white female, with four prior fibular stress fractures, suffered a left femoral shaft fracture during the 13th mile of a half-marathon. The fracture was successfully internally fixed. Biochemical studies showed no metabolic abnormality. Bone mineral density of the lumbar spine, femoral neck, tibia, and fibula were below the mean for both eumenorrheic and amenorrheic female athletes. Exercise-associated amenorrhea is a medical problem that may have serious implications for both competitive and high-intensity recreational female athletes.