Trials to determine efficacy of pneumococcal vaccine prior to its licensure resulted in estimates of efficacy of 76%–100%. However, the populations studied in these trials were dissimilar to the population in this country at high risk of morbidity and mortality due to pneumococcal infection. Estimates based on data obtained in this country after licensure of the vaccine suggest that the efficacy in certain groups may be low: ⩽0% in children two to 10 years of age and 49% (95% confidence interval, ⩽0%–75%) in persons older than 10 years of age.