In the retrospective cohort study based on record linkage, the authors studied a cohort of persons born in 1900-1930 (n = 144,627), who had lived in the same rural location at least from 1967 to 1980. Estimates for fluoride concentrations (median, 0.1 mg/liter; maximum, 2.4 mg/liter) in well water in each member of the cohort were obtained by a weighted median smoothing method based on ground water measurements. Information on hip fractures was obtained from the Hospital Discharge Registry for 1981-1994. No association was observed between hip fractures and estimated fluoride concentration in the well water in either men or women when all age groups were analyzed together. However, the association was modified by age and sex so that among younger women, those aged 50-64 years, higher fluoride levels increased the risk of hip fractures. Among older men and women and younger men, no consistent association was seen. The adjusted rate ratio was 2.09 (95% confidence interval: 1.16, 3.76) for younger women who were the most exposed (>1.5 mg/liter) when compared with those who were the least exposed (< or =0.1 mg/liter). The results suggest that fluoride increases the risk of hip fractures only among women.