Thiazide diuretics reduce urinary calcium and may inhibit bone resorption, and hence may help to attenuate age-related bone loss and to lower the risk of osteoporotic fracture. We followed 83728 women, who were 36-61 years of age at baseline in 1982, for 10 years with biennial mailed questionnaires on which they reported incident fractures, use of thiazide diuretics, and other medical behavioral information. From descriptions of fracture sites and circumstances, 251 hip (proximal femur) and 1594 forearm (distal radius) fractures were identified as low or moderate trauma events. After controlling for age, body mass index, menopausal status, postmenopausal hormone use, cigarette smoking and dietary factors, we observed a statistically significant 22% reduction in the risk of forearm fractures among current thiazide users compared with women who reported no thiazide use. Risk appeared to decline with longer duration of use, reaching a 37% reduction in risk among women who had been using thiazides for 8 or more years. For hip fractures, thiazide use was protective among the postmenopausal women (relative risk = 0.69, 95% confidence interval 0.48-0.99). We conclude that the potential benefit of thiazide diuretics for osteoporosis should be considered when prescribing antihypertensive treatment.