We have determined the level of persisting pneumococcal antibody in a group of nephrotic children vaccinated by us 5 years ago. Of the 19 vaccinated children, 2 have died and 1 has moved away. Sera from the remaining 16 patients were examined by radioimmunoassay to determine the antibody response to 11 of the 14 types contained in the polyvalent pneumococcal vaccine. The lowest protective level of geometric mean titre (GMT) of antibody in our laboratory is 300 ng antibody nitrogen per millilitre. 56% (9/16) of the patients showed adequate GMT 5 years after vaccination. All 9 patients had minimal change nephrotic syndrome. 44% (7/16) of the children had a GMT less than 300 ng antibody nitrogen per millilitre. 3 of these patients had focal sclerosis, 3 had membranoproliferative glomerulonephritis, and 1 patient had IgM nephropathy. Of these 7 patients, 1 with the lowest GMT (23 ng antibody nitrogen per millilitre) developed pneumococcal peritonitis. During this same period, in 20 other unvaccinated nephrotic patients followed continuously from 1976 to 1981, 7 cases of pneumococcal peritonitis occurred (p less than 0.05). Additionally, 1 unvaccinated child died of pneumococcal sepsis. Our study indicates that patients with minimal change nephrotic syndrome continue to maintain adequate amounts of antibody, but those with disease other than minimal change nephrotic syndrome are unable to maintain an adequate level of antibody.