In 191 patients with mesangial IgA nephropathy, GFR was determined as clearance of 51Cr-EDTA. 86 (45%) of them had subnormal renal function 7.3 +/- 4.6 years after renal biopsy. The change in GFR was followed in 153 patients with repeated determinations of 51Cr-EDTA clearance. 50.3% of the patients had a loss of more than 1.1 ml/min/year, which we regard as pathological. The markers of progressive disease were: male sex, high output of urinary protein, severe histological lesions and presence of hypertension. Even patients lacking these markers had a significantly increased incidence of progressive disease. Of 93 patients, with initially normal GFR, 32% will have a subnormal GFR within five years and 25% will develop end-stage renal failure within 20 years. In 38 patients with six or more determinations of 51Cr-EDTA clearance, the predictive value of the first four determinations was calculated. Of 26 with a decrease of more than 1.1 ml/min/year, 13 (50%) developed subnormal GFR during follow-up, while 11 of 12 (91.7%) with a decrease of less than 1.1 ml/min/year (P less than 0.05) remained normal. This shows that repeated determinations of GFR with an accurate method will predict the final outcome early in the disease. We also confirmed that single or repeated determinations of clearance of creatinine are of little value in separating a normal GFR from a slightly decreased one, but more reliable in detecting a markedly reduced GFR.