Renal function and fluoride excretion have been studied in 38 children. The children were divided into three groups according to their glomerular filtration rate: normal (92 to 136 mL/min/1.73 m2 of body surface area [BSA]), low (less than 92 mL/min/1.73 m2 BSA, and super-normal (greater than 136 mL/min/1.73 m2 BSA). Standard clearance technique with infusion of inulin and p-aminohippuric acid during water diuresis was used. Mean renal fluoride clearance was 45.0 +/- 9.8 (SD) mL/min in the group of children with normal glomerular filtration rates and 31.4 +/- 8.8 mL/min in the group with low glomerular filtration rates. This difference was statistically significant. There was a close linear relationship between renal fluoride clearance and glomerular filtration rate, urinary flow, and free water clearance. The fractional fluoride excretion did not differ between the groups. About 60% of the filtered fluoride was reabsorbed. No evidence for tubular secretion exceeding the reabsorption could be found. The results suggest that children have lower renal fluoride clearance rates than adults and indicate that a moderate impairment of the renal function could lead to increased retention of fluoride.