Based on statistical analysis of data in 186 children, a formula was derived which allows accurate estimation of glomerular filtration rate (GFR) from plasma creatinine and body lenght (GFR(ml/min/1.73 sq m) = 0.55 length (cm)/Per (mg/dl). Its application to clearance data in a separate group of 223 children reveals excellent agreement with GFR estimated by the Ccr (r = .935) or Cin (r = .905). This formula should be useful for adjusting dosages of drugs excreted by the kidney and detecting significant changes in renal function.