Of 104 patients with idiopathic mesangiocapillary glomerulonephritis studied for at least two years, 69 patients had type I disease and 35 had type II. Forty-five patients were children, and 59 were adults. Type II mesangiocapillary glomerulonephritis was more common in children than in adults, but no other clinical feature distinguished the two types at onset. Complement studies revealed that patients with type II had lower serum C3 concentrations and more frequently showed C3-splitting activity (C3 nephritic factor) in the serum. Children had hypertension or a lowered glomerular filtration rate less frequently at onset than did adults, but children had a higher incidence of a hematuric onset; C3 nephritic factor was also more frequent in the children. During a follow-up period of two to 21 years (mean eight years), only seven patients (five with type I and two with type II) showed clinical remission, whereas 38 percent of patients with type I and 49 percent of patients with type II died or required dialysis; a further 23 percent of patients with type I and 16 percent of patients with type II had continuing disease and reduced glomerular filtration rate. Only the presence and persistence of a nephrotic syndrome in type I predicted renal failure. In both types, the presence of sclerosis or crescents in the initial renal biopsy specimen was associated with a poorer prognosis, but no other feature was of major prognostic value.