We retrospectively examined the effect of nephrectomy on renal function in 55 living related donors. Renal function was measured with 131iodine-orthoiodohippurate scans. All patients were studied preoperatively, and 1 week and 1 year postoperatively. In 20 patients 10-year followup was available. Compensatory hypertrophy was complete 1 week postoperatively: effective renal plasma flow of the remaining kidney was 32.5% higher than preoperatively. The increase remained stable for at least a year. The degree of compensatory hypertrophy was significantly greater in male patients (46.9% after 1 week) than in female patients (26.7%). Compensatory hypertrophy occurred in all age groups studied and it was most pronounced in patients less than 30 years old. In the patients followed for 10 years effective renal plasma flow decreased from 387.7 ml. per minute 1 week after nephrectomy to 367.4 ml. per minute at 10 years. This result is similar to the decrease seen in the normal population. According to our results, renal donation by living related persons does not lead to long-term decrease in renal function.