Reduction of the renal parenchyma in experimental animals can cause glomerulosclerosis in the single kidney and lead sometimes to functional failure. Data on the influence of nephrectomy or agenesis on the later fate of the patient in human medicine are controversial. The authors examined a total of 40 patients with single kidneys. In 23 agenesis was involved (15 boys, 8 girls) aged 6-22 years (mean = 11.9, s = 4.3). In 17 nephrectomy was performed (9 boys, 8 girls); time interval after nephrectomy 1-26 years (mean = 7.8, s = 3.9). Three quarters of all patients excreted increased amounts of albumin (detected as microalbuminuria by immunoturbidometry). The glomerular filtration rate (GFR) was slightly reduced in about one third of the patients but none suffered from renal insufficiency. In one half of the group the authors detected a slightly impaired concentrating capacity with an increased sodium fraction. In one third marginal diastolic hypertension was recorded. In half the group the single kidney was hypertrophic. Regression analysis did not reveal deterioration of the GFR in relation to age but revealed increasing renal hypertrophy in relation to height with accelerated hypertrophy during puberty. There was also a significant drop of the systolic blood pressure in relation to age in patients with agenesis. In none of the other parameters differences were found between the group with agenesis and nephrectomy. Patients with a single kidney deserve prolonged ambulatory follow up. In the investigated group the single kidney influenced in a substantial way also the selection of occupation.