From the practices of approximately two thirds of the orthopedic surgeons in King County, Washington, the authors identified white women ages 50 to 74 who had sustained a fracture of the hip or forearm between 1976 and 1979. The women were interviewed concerning factors associated with levels of endogenous estrogens. Their responses were compared to those of a random sample of white female residents of King County in the same age range. The risk of hip fracture was elevated in thin women and in those who smoked cigarettes, particularly among nonusers of estrogen. Neither weight nor smoking affected the risk of forearm fracture in estrogen users, although among nonusers smoking increased the risk, particularly among those who were thin. The beneficial effect of estrogen use in preventing both types of fracture varied considerably according to the woman's weight and smoking status, being greatest in thin women who smoked cigarettes and near zero in heavy nonsmokers. Body weight and cigarette smoking history offer important information concerning the probable degree of protection against fracture afforded by estrogen use.