The aim was to study the effect of ingested fluoride in patients with chronic renal failure (CRF). Serum fluoride concentrations were measured in 104 subjects, who formed three groups: nondialyzed CRF, dialyzed CRF, and a control group. The iliac bone fluoride was measured in 20 subjects. Serum, urine and water fluoride concentrations were determined by the method of Fuchs. The bone calcium and fluoride levels were measured by non-destructive neutron activation analysis. Serum concentrations of fluoride in the nondialyzed and dialyzed CRF groups and the control group were 46.4±12.6 (n=14), 28.2±12.3 (n=40), and 7.92±2.64 (n=50) ug/L respectively. Serum fluoride levels in both patient groups were higher than in the control group (p<0.05). The serum fluoride level of the nondialyzed patients correlated with their creatinine clearance (r=0.379), indicating that fluoride excretion from the body was dependent on renal function. In dialyzed patients, the serum fluoride level decreased from 28.2±12.3 to 14.7±4.57 ug/L (n=40) 4 hours after a single hemodialysis. The fluoride concentration of the tap water was 53.l±15.9 ug/L (n=8). The fluoride content of iliac bone obtained from dialyzed patients was 4640±106 d weight and were approximately 10 times higher than the control 440±35 ug dry weight (p<0.001) suggesting that skeletal fluorosis occurs in patients with CRF using fluoridated water. The bone calcium of the dialyzed patients and the control group were 119±13 mg/g and 125±29 mg/g (p=ns) respectively. Fluoride excretion was dependent on renal function with the serum fluoride concentration increasing in both the nondialyzed and dialyzed patients. Bone fluoride levels increased over 10 times those of the control group. This strongly suggests that the high fluoride contents in the bone is one of the factors inducing renal osteodystrophy. These results therefore strongly indicate that fluoridation of drinking water will be toxic for patients having decreased renal function.