The ability of the Schwartz formula (CSCH) to estimate glomerular filtration rate (GFR) accurately was investigated in children with renal disease. 125Iodine-iothalamate clearance (CIO) was used as the reference standard for measuring GFR. Data from 176 CIO studies performed on 133 children (aged between 1 and 18 years) were compared with the simultaneous estimation of GFR by CSCH. The overestimation of GFR by CSCH was inversely proportional to the level of renal function. When CIO was > 90 ml/min per 1.73 m2, CSCH overestimated GFR by only 0.1% +/- 3%, but when CIO was < or = 15 ml/min per 1.73 m2, CSCH overestimated GFR by 164% +/- 42%. When renal function is normal or mildly reduced (GFR > 50 ml/min per 1.73 m2), CSCH overestimated CIO by only 10.3 +/- 3.0%, compared with 90.3 +/- 14.5% when renal function was moderately to severely curtailed (GFR < or = 50 ml/min per 1.73 m2). We conclude that CSCH is valid in predicting GFR only in children with normal renal function and mild insufficiency.