A prospective study was initiated in 1976 to investigate runners who are at risk for incurring stress fractures and how these fractures can be prevented. Fifty-one runners incurred 57 stress fractures. Tibial fractures were most common (25), followed by fibula (12) and metatarsal (8). Seven runners had previously sustained stress fractures, and six developed two stress fractures simultaneously. Five women over 30 years old had pelvic stress fractures. Stress fracture development was positively correlated with the presence of pes planus, weekly training distances greater than 20 miles, hard training surfaces, and training regimen modifications. The incidence did not correlate with generalized musculoskeletal laxity or tightness. Forty-four of 51 patients had initially positive roentgenograms. Five of five bone scans were positive. The average duration of rest before running was resumed was 7.4 weeks.