This report critically reviews 15 case-control studies that disagree about whether patients who fracture their hip are significantly more osteoporotic than persons of similar age who do not. The most rigorously designed studies observed less bone mass in the hips of patients with fractures than In the hips of control subjects, but the differences were usually small and overlapping. Measurements at other sites in the skeleton did not consistently find differences. Those studies that protected against ascertainment bias generally found smaller differences than studies that did not. Patients with hip fractures do not appear to be distinctly more osteoporotic than persons of similar age. Therefore, factors besides bone mass, such as a tendency to fall, may be important determinants of which elderly persons will have fractures; thus, measurements of bone mass might not be a reliable way to identify those at greatest risk of hip fracture.