The authors investigated the persistence of anticapsular pneumococcal antibodies in 21 subjects one decade after administration of a single dose of a polyvalent pneumococcal polysaccharide vaccine. Fourteen vaccinees received a hexavalent vaccine composed of the polysaccharides of capsular types 1, 3, 4, 7F, 8, and 12F; four vaccinees received an octavalent vaccine consisting of these six polysaccharides and also those of capsular types 14 and 19F; and three vaccinees received a nonavalent vaccine that also included type 5 capsular polysaccharide. Antibody was measured by radioimmunoassay. The authors detected persistently elevated anticapsular antibody levels among more than one half of vaccinees who developed a significant rise in antibody 1 month following immunization one decade after administration of pneumococcal polysaccharide vaccine when these levels were compared to prevaccine levels for pneumococcal capsular types 4, 7F, and 8. This finding was not the case with pneumococcal types 1, 3, 12F, 14, and 19F; less than two fifths of vaccinees maintained increased levels of anticapsular antibody to these types one decade after administration of pneumococcal vaccine. Geometric mean anticapsular antibody levels for types 7F and 8 only were significantly higher one decade after vaccine administration compared with the levels before immunization (t-test, p less than 0.01).