Children 12-18 months old were randomized to receive one dose of a conjugate heptavalent pneumococcal vaccine, two doses of the same vaccine, or one dose of a 23-valent native polysaccharide vaccine. Before immunization, pneumococci included in the conjugate vaccine were isolated from 24% of the children, and an antibiotic-resistant pneumococcus was isolated from 22% of the children. The vaccines had no effect on carriage of non - vaccine-type pneumococci. In contrast, there was a significant reduction in carriage of vaccine-type pneumococci 3 months after one dose and 1 month after a second dose of conjugate vaccine (from 25% to 9% and 7%, respectively; P < .001). No effect was seen after vaccination with the nonconjugate vaccine. One year after immunization, carriage of antibiotic-resistant vaccine-type pneumococci in children receiving conjugate vaccine was lower than that in children receiving the nonconjugate vaccine (4% vs. 14%, P = .042). Conjugate pneumococcal vaccines may reduce spread of pneumococci in the community.