The clinical features and renal histology of twelve chinldren who developed nephrotic syndrome in the first year of life were studied. Six suffered from microcystic disease and six from primary mesangial cell proliferation and/or sclerosis. A consistent family history, premature birth, large placenta pressence of other congenital abnormalities, onset in the first two months of life and lower plasma albumin level all suggested microcyste disease, but the most reliable distinction was histological. All microcystic children died within two years, whereas four with primary mesangial disease survived indefinitely. Corticosterid and immunosuppressive herapy failed to help either group and most deaths were due to sepsis, especialy with E. coli.