To test the hypothesis that osteoporosis, which results partly from long-term estrogen deficiency, is associated with a lowered risk of breast and endometrial cancer, a population-based cohort study was performed in Sweden. Some 18,000 women were followed through record-linkages after a first hip fracture, on average for almost six years, to ascertain the incidence of breast and endometrial cancer. Expected numbers were derived from the general population. Overall, 253 cases of breast cancer were observed cf 300 expected (standardized incidence ratio [SIR] = 0.84; 95 percent confidence interval [CI] = 0.74-0.95). Forty-eight cases of endometrial cancer were found cf 55 expected (SIR = 0.87, CI = 0.54-1.16). There was no clearcut pattern of breast cancer risk by age at diagnosis of hip fracture, follow-up time, or calendar period. Our results are consistent with the a priori hypothesis that long-term estrogen deficiency is associated with a reduced risk of developing breast cancer as well as endometrial cancer.