Renal haemodynamics and the pattern of urinary protein excretion were studied in 38 children (21 boys, 17 girls) with biopsy-proven IgA nephropathy (IgAN), 0.4-16.8 (median 5.3) years after onset of the disease. Glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were evaluated by clearances of inulin and para-aminohippuric acid. Serum and urinary albumin, IgG and beta 2-microglobulin (beta 2 mu) were determined and the excretion rates, clearances, and fractional clearances were calculated. The patients were grouped according to the type and the amount of proteinuria. Mean GFR and ERPF were significantly decreased (107 +/- 3 and 580 +/- 17 ml/min per 1.73 m2, respectively) versus controls (119 +/- 2 and 627 +/- 14 ml/min per 1.73 m2, respectively). Grouped according to albumin excretion rates, non-albuminuric patients had normal GFR, while mean GFR was reduced in patients with micro-albuminuria (106 +/- 3 ml/min per 1.73 m2) and albuminuric patients (92 +/- 7 ml/min per 1.73 m2). IgG excretion increased with increasing albuminuria, but the selectivity index was lower in albuminuric patients than in patients with micro-albuminuria. Albuminuric patients had also higher blood pressure than those with micro-albuminuria. beta 2 mu excretion did not discriminate between patients with impaired renal function. The results suggest that childhood IgAN is not a benign kidney disease. After a median duration of 5 years of the disease a number of children had impaired renal function. Mean GFR was reduced most in the albuminuric patients but was also decreased in micro-albuminuric patients, indicating that micro-albuminuria may be a predictor of more severe disease.