Most studies evaluating renal function post-renal transplantation in children have used serum creatinine (S(Cr)) or estimates of its clearance (C(SCH)). When renal function is impaired both S(Cr) and the C(SCH) overestimate glomerular filtration rate (GFR), especially during cyclosporine therapy. This study measured GFR in 64 children (age range: 4-19 years) with stable renal function who received renal allografts at the Childrens Medical Center of Dallas, 31 from live related donors (LRD) and 33 from cadaveric donors (CAD). 125I-iothalamate clearance (C(IO)) was used as the reference standard for measuring GFR. Data from 100 C(IO) studies, were analyzed and results reported as mean +/- S.E.M. C(IO) performed during the first year after renal transplantation in 23 children who received allografts from LRD was 72.4+/-5.5 ml/min per 1.73 m2 compared to 50.4+/-7.4 ml/min per 1.73 m2 in 18 children who received allografts from CAD (p<0.05). Beyond the first year post-renal transplantation there was no difference in C(IO) between LRD and CAD allografts. When S(Cr) was compared to C(IO), the relationship was nonlinear. C(IO) was also compared to the simultaneous estimation of creatinine clearance by C(SCH). The overestimation of GFR by C(SCH) was inversely proportional to the level of renal function. When renal function was normal or mildly reduced (C(IO) > 50 ml/min per 1.73 m2), C(SCH) closely approximated C(IO). When renal function was moderately to severely curtailed (C(IO) < or = 50 ml/min per 1.73 m2), C(SCH) overestimated C(IO) by 43.6+/-5.6%. The study concludes that in children with renal transplant: 1) C(IO) is higher in allografts from LRD compared to CAD kidneys only in the first 12 months following renal transplantation; 2) S(Cr) is a poor predictor of C(IO); and 3) C(SCH) consistently overestimates GFR children following renal transplantation unless renal function is normal or only mildly decreased.