We compared the adverse reaction rates and antibody responses when influenza and pneumococcal vaccines were given simultaneously and separately. We used a double-blind, placebo-controlled protocol in which the participants, healthy university students and staff between 18 and 26 years of age, were randomly assigned to the different vaccination groups. Simultaneous administration of the two vaccines did not result in any increase in local or systemic reactions, except for transient myalgia. Study participants who received the vaccines simultaneously had pneumococcal antibody responses comparable with those of participants who received pneumococcal vaccine alone, although the response rate for both groups was low. There was also no difference between the simultaneous-administration group and the group that received influenza vaccine alone in antibody response to the influenza A/Texas antigen. Antibody responses to the influenza A/Brazil and B/Hong Kong antigens were adequate for both groups, although the group that received influenza vaccine alone achieved higher postvaccination antibody titers. We conclude that simultaneous administration of influenza and pneumococcal vaccines is as safe and effective as giving either vaccine alone.