The immunogenicity and potential for disease modification of pneumococcal polysaccharide vaccine in systemic lupus erythematosus were evaluated in a controlled, double-blind study. Forty patients were randomly chosen to receive an intramuscular injection of either vaccine or placebo. Changes in mean antibody concentrations (nanograms antibody nitrogen per milliliter serum) to 12 type-specific pneumococcal capsular antigens from prevaccination to one month after vaccination were 177 to 1045 in the vaccine (P less than 0.001) and 164 to 153 in the placebo-treated patients. In the month after vaccination, neither vaccine nor placebo-treated patients had a significant change in lupus disease activity as assessed by a composite clinical, laboratory, and serologic index. We conclude that patients with systemic lupus erythematosus can be successfully immunized with pneumococcal vaccine without detectable alterations of the underlying disease.