A number of clinical, laboratory and pathologic parameters were assessed for their prognostic significance in 200 children aged less than 15 years with IgA nephropathy, who had shown normal renal function at the time of initial biopsy and were followed for more than 2 years thereafter. After a mean follow-up period of 5.0 years from the initial biopsy, 93 patients had no demonstrable abnormality, 76 had minor urinary abnormalities, 21 had persistent heavy proteinuria and 10 had developed chronic renal impairment. A poor outcome was found to be correlated with heavy proteinuria at biopsy, diffuse mesangial proliferation, a high proportion of glomeruli showing sclerosis, crescents or capsular adhesions, the presence of moderate or severe tubulointerstitial changes, and the presence of subepithelial electron-dense deposits and lysis of the glomerular basement membrane by electron microscopy. The percentage of glomeruli displaying crescents, sclerosis and adhesions appeared to be the most reliable prognostic indicator. Nine of the 27 patients (33%) in whom greater than or equal to 30% of glomeruli showed crescents, sclerosis and adhesions developed chronic renal impairment, and only 14% of these patients had normal urine at follow-up. In contrast, only 1 of the 173 patients in whom less than 30% of glomeruli showed such lesions developed chronic renal impairment (p less than 0.001) and 51% of these patients showed complete remission at follow-up (p less than 0.001). These results demonstrate that an accurate prediction of the outcome based on the initial renal biopsy findings is possible early in the course of children with IgA nephropathy.