The clinical and histologic features of 81 patients with IgA nephropathy were analyzed. Azotemia was present in 32 per cent of the patients, proteinuria was present in 88 per cent, and gross or microscopic hematuria was present in all of the patients tested. The median age of histologic diagnosis was 27 years. The median age at onset of clinical signs was 20 years. There was no increased incidence in any HLA-A or -B antigen within the patient population over our control population. All patients had glomerular mesangial IgA deposition (by definition) greater than or equal to IgG or IgM. Histologic changes were quantitated and ranged from normal to necrotizing and/or crescentic glomerulonephritis. Many patients (48 per cent) had mild or moderate generalized glomerlular hypercellularity. Nonparametric statistical analysis showed strong correlations among patient age at histologic diagnosis, creatinine, proteinuria, global glomerular sclerosis, and interstital fibrosis. Our analysis suggests that IgA nephropathy is an indolent disease generally beginning in childhood. It is a cause of renal insufficiency in a significant number of patients. Interpretation of this series and other reported studies suggests that most cases of IgA nephropathy in the United States are best considered idiopathic but that hereditary and secondary forms may exist.