Bone mineral density (BMD) of the spine and femoral neck was measured in a random stratified sample of 3222 perimenopausal women aged 47-59 years. A total of 969 women had used fluoridated drinking water (1.0-1.2 mg/l) for over 10 years. These women were compared with 2253 women with low levels of fluoride in drinking water (< 0.3 mg/l). BMD of the spine was significantly higher in the fluoride group than in the non-fluoride group (1.138 +/- 0.165 vs. 1.123 +/- 0.156 g/cm2, P = 0.026). Femoral neck BMDs did not differ between the groups. When the BMD values were adjusted for confounding factors (age, weight, menopausal status, calcium intake, physical activity level, deliveries, alcohol consumption and estrogen use), the differences between the groups increased (P < 0.001 for the spine and P = 0.004 for the femoral neck, respectively). There was no significant difference between the groups in the prevalence of self-reported fractures sustained during 1980-1989. We propose that the fluoridation of drinking water has a slight increasing effect on axial BMD in women in low fluoride areas.