Abstract After an acute attack of otitis media 827 children aged 3 months to 6 years were assigned randomly to receive either 14-valent pneumococcal polysaccharide vaccine or a control vaccine ( Hœmophilus influenzœ type b capsular polysaccharide). In children older than 6 months serum antibody responses to most of the vaccine polysaccharides were satisfactory. The response to type 6A was poor. Correspondingly, no clinical protection was seen below 6 months of age or against otitis media caused by group 6 pneumococci. Among the children more than 6 months old, the first 6 months after vaccination saw significantly (p<0·001) fewer attacks caused by the pneumococcal types represented in the vaccine (group 6 excepted) in those who received the pneumococcal vaccine than in those who received the control vaccine. Protection against type 19F was statistically significant (p<0·01). The overall protective efficacy was 58%—somewhat better in children older than 2 years than in those younger. Previous attacks of otitis caused by pneumococci did not influence the protective efficacy of the vaccine.