A combined retrospective and prospective study of 37 patients with IgA-IgG nephropathy was conducted with the purpose of identifying those clinical, laboratory, and biopsy features associated with impaired renal function and progression of renal disease. At initial evaluation, 11 patients had impaired renal function and 26 had normal renal function. In five patients (only one of whom had normal renal function at onset), end-stage renal disease developed. Features associated with disease progression were male sex, a long clinical course, urine protein level of greater than 3.5 g/24 hr, hypertension, and glomerulosclerosis. A statistically significant increase in serum IgA levels was found at follow-up in the study population, compared with the levels in control subjects, but no difference was noted between those patients with normal and those with abnormal renal function. In addition, immunofluorescent studies on renal biopsy specimens suggest that activation of the alternate complement pathway predominates.