Fluoride in plasma, urine and bone tissue ash were estimated using a fluoride-ion electrode in 20 control persons (CP), 32 patients with compensated chronic renal failure (CRFP) and 59 patients in RDT (RDTP). The increase in plasma fluoride (CP: 2.4 +/- 1.4, CRFP: 6.5 +/- 2.2, RDTP: 12.3 +/- 4.5 mumol/L) and bone fluoride (CP: 55.1 +/- 31, CRFP: 99.9 +/- 31.2, RDTP: 339.1 +/- 150.6 mumol/g) significantly correlated with the decrease of residual glomerular filtration rate (RGFR), in RDT with the number of haemodialysis, so that the maximum increase in fluoride was found in completely anuric patients (379.7 +/- 153 mumol/g). The increase in fluoride retention was intensified by body retention of considerable amounts of fluoride each dialysis (the fluoridated dialysate increased the post-dialysis plasma fluoride by 195%). Development of bone fluorose known to develop with a bone fluoride greater than 180 mumol/g was not found in any of 40 iliac crest trephine bone biopsy specimens. No correlation was found between laboratory and histological findings of renal osteodystrophy and plasma or bone fluoride. No patient developed spontaneous fractures even after 11 years of using fluoridated dialysate. In conclusion, this report indicates that fluoride might have a protective effect against the progression of renal osteodystrophy in patients with high retention values. The longer the exposure of RDT patients to the fluoridated dialysate, the greater the bone fluoride concentration.