The authors investigated whether the incidence of first hip fracture, an indicator of osteoporosis, is lower in breast cancer patients, who are assumed to have higher levels of endogenous estrogens, than in other women. A population-based Swedish cohort comprising 9,673 women with invasive breast cancer diagnosed from 1958 through 1983 was followed up with respect to the occurrence of a first hip fracture during the period 1965-1983. Overall, the number of observed acute fractures (n = 387) was slightly higher than expected (n = 348.6) (standardized incidence ratio (SIR) = 1.1, 95% confidence interval (CI) 1.0-1.2). Risk for trochanteric fractures was slightly higher than expected (SIR = 1.2, 95% CI 1.0-1.4), but risk for cervical fractures was not (SIR = 1.0, 95% CI 0.9-1.1). Risk for trochanteric fracture decreased with increasing age at breast cancer diagnosis, reaching standardized incidence ratios close to unity after the age of 70 years. Duration of follow-up appeared to be unrelated to the risk of either type of fracture. The authors conclude that the incidence of first hip fracture is not lower in breast cancer patients than in other women.