A total of 827 infants and children three months to six years of age were vaccinated randomly after an attack of otitis media, with either the 14-valent pneumococcal vaccine or a control vaccine against Hemophilus influenzae type b. Clinically acute attacks of otitis media that occurred greater than or equal to 14 days after vaccination were analyzed by culture of the middle ear fluid. Children who had received the pneumococcal vaccine at the age of seven to 83 months experienced within the first six months after vaccination significantly fewer attacks of otitis media caused by Streptococcus pneumoniae than did children in the control group (50% overall reduction, p less than 0.01). The reduction of attacks was specific to those pneumococcal types/groups present in the vaccine that induced a good serum antibody response; the specific protection indicated for vaccine types/groups other than 6 (to which the antibody response was very poor) was 67% (P less than 0.001). No or very little protection was seen in infants younger than seven months or in any children later than six months after vaccination.