We measured bone mineral density (BMD) at the lumbar spine (LS-BMD) and ultradistal radius (UDR-BMD) in 42 postmenopausal normal women and in 108 postmenopausal osteoporotic women (55 with vertebral fracture, 34 with Colles' fracture, and 19 with both fractures). By receiver operating characteristic analysis, LS-BMD was better than UDR-BMD (P less than 0.01) as an indicator of vertebral fracture; the converse was true for Colles' fracture (P less than 0.01). Although UDR-BMD and LS-BMD were lower in each of the three fracture groups than in controls (P less than 0.01), the pattern of bone loss differed (P less than 0.001, analysis of variance): with vertebral fracture, LS-BMD decreased relatively more than UDR-BMD; with Colles' fracture, UDR-BMD decreased relatively more than LS-BMD; and with both fractures, decreases in LS-BMD and UDR-BMD were similar. We conclude that both types of fracture are caused by excessive bone loss but the difference in bone loss at the two sites is a major factor in determining which will fracture.