Renal functional impairment due to the damaging effect of hyperfiltration of the remnant kidney was evaluated in patients who had been uninephrectomized in childhood or had unilateral agenesis. Sixteen patients (8 male, 8 female) a mean of 7.1 years (1-26 years) postnephrectomy were evaluated. Blood pressure values, clearances of creatinine (Ccr) and phosphorus (Cp), urine osmolality, total protein excretion and NAG (N-acetyl-beta-d-glucosaminidase) excretion were determined. Ccr values were slightly but not significantly lower (90.390 +/- 11.223 ml/min) than those of control subjects (110.818 +/- 7.755 ml/min). Daily urinary protein excretion adjusted for body surface was significantly higher in the study group (p < 0.05). Urinary NAG excretion measured with a spectrophotometric method and described as the Cr ratio was significantly higher in the patients than the control subjects (p < 0.05). It was concluded that the damaging effects of hyperperfusion not only involve the glomerulus but also the proximal and distal tubulus.