At the time of kidney biopsy the pattern of urinary protein excretion (UPE) and renal function were studied in 54 patients (age 16-62 years) with IgA nephropathy (IgAN). Serum and urinary albumin (alb), IgG, beta-2-microglobulin and creatinine were analysed, and excretion rates (UV) and clearances were calculated. The glomerular filtration rate (GFR) was determined by plasma 51Cr-EDTA clearance (51Cr-EDTA) and by 24-hour creatinine clearance (C-Cr 24 h). Glomerular mesangial (volume expansion and cell proliferation), tubulo-interstitial (fibrosis and inflammation) and vascular lesions were classified semiquantitatively on a five-degree scale, and the percentage of glomeruli showing global sclerosis, segmental sclerosis and cellular crescents was calculated. One third of our patients had reduced renal function, three patients uremia and 70 per cent of the patients overt albuminuria. The mean GFR was reduced in microalbuminurics and further decreased in albuminurics and nephrotics. A lower GFR and more proteinuria were found in the patients with more advanced morphological lesions also when the uremic patients were excluded. Segmental glomerular sclerosis correlated with GFR as well as with UalbV and UIgGV, while global sclerosis correlated only with GFR. UalbV and UIgGV also correlated with the extent of interstitial damage but not with mesangial lesions. In summary an accurate determination of GFR and UPE at the time of the kidney biopsy may give an indication of the extent of renal damage. A lowered GFR was also found in mild proteinuria.(ABSTRACT TRUNCATED AT 250 WORDS)