Membranous glomerulonephropathy (MGN) in South African black and mixed race children with the nephrotic syndrome is much commoner than in First-World countries. In this survey of 388 nephrotic children MGN was found in 51.9% of black and 20.9% of mixed race boys, and 25% of black and 5.6% of mixed race girls respectively, but was not present in 53 white and Asiatic nephrotic children. Aetiological or associated factors were documented in 84%: hepatitis B virus infection in 73%, congenital syphilis in 6% and systemic lupus erythematosus, D-penicillamine toxicity and Salmonella infective endocarditis in 1 case each. The prognosis depends on the cause and is much better than for adults with idiopathic MGN. After an average follow-up period of 4.5 years the overall remission rate was 78% and mean time to remission 30 months. One patient with syphilitic MGN died 15 years later; 3 patients are in mild renal failure. Corticosteroids and other immuno-suppressive therapy were ineffective and may do harm. The frequent occurrence of MGN is related to the high prevalence of predisposing infections in the affected population groups, and socio-economic rather than ethnic factors are important.