The influence of simultaneous vaccination with Pneumovax and tetanus toxoid on plasma human immunodeficiency virus (HIV) RNA and CD4 cell counts was assessed in patients with HIV infection. Six weeks after immunization plasma HIV RNA did not differ significantly from baseline values. A slight decline in CD4 cell counts after vaccination was detected. There was no correlation between the serological responses to pneumococcal vaccination and baseline CD4 cell counts, changes in HIV RNA or CD4 cell counts. In conclusion, immunization did not induce an increase in viral replication detectable 6 weeks after immunization.