OBJECTIVE--To test the effect of water fluoridated to 1 ppm on the incidence of hip fractures in the elderly. DESIGN--Ecological cohort. SETTING--The incidence of femoral neck fractures in patients 65 years of age or older was compared in three communities in Utah, one with and two without water fluoridated to 1 ppm. PATIENTS--All patients with hip fractures who were 65 years of age and older over a 7-year period in the three communities, excluding (1) those with revisions of hip fractures, (2) those in whom the hip fracture was anything but a first diagnosis, (3) those in whom metastatic disease was present, or (4) those in whom the fracture was a second fracture (n = 246). OUTCOME MEASURE--Rate of hospital discharge for hip fracture. RESULTS--The relative risk for hip fracture for women in the fluoridated area was 1.27 (95% confidence interval [CI] = 1.08 to 1.46) and for men was 1.41 (95% CI = 1.00 to 1.81) relative to the nonfluoridated areas. CONCLUSIONS--We found a small but significant increase in the risk of hip fracture in both men and women exposed to artificial fluoridation at 1 ppm, suggesting that low levels of fluoride may increase the risk of hip fracture in the elderly.