Eighteen episodes of community-acquired bacterial pneumonia were diagnosed in 13 patients among 336 with the acquired immunodeficiency syndrome (AIDS) cared for at Memorial Sloan-Kettering Cancer Center since 1979. Bacterial pathogens isolated in 16 of 18 episodes were Haemophilus influenzae in 8, Streptococcus pneumoniae in 6, group B streptococcus in 1, and Branhamella catarrhalis in 1. Eight episodes were presumed Pneumocystis carinii pneumonia until cultures obtained at bronchoscopy confirmed a bacterial cause. Specific antibacterial therapy was curative in 16 of 18 episodes; 2 patients died. Given an estimated yearly incidence of pneumococcal pneumonia in the general population of 2.6/1,000, 1.09 cases were expected in our patients with AIDS; we saw 6 (p = 0.001), for an attack rate of 17.9/1,000. Bacteria associated with B-cell defects should be anticipated when formulating empiric antibiotic therapy, pending a definitive diagnosis, for pulmonary infiltrates in patients with AIDS.