Bacterial pneumonia is significantly more common in persons who are HIV-infected than in the general population and is most common among injection drug users and in persons with advanced HIV disease and immunosuppression. The clinical features of bacterial pneumonia are similar to those in HIV-seronegative persons, but bacteremia is more common. When a pathogen is identified, Streptococcus pneumoniae is consistently the most common, occurring in 20% to 70% of cases. Haemophilus influenzae, Staphylococcus aureus, Escherichia coli, and other gram-negative organisms are mainly responsible for the remainder of bacterial pneumonia episodes in the United States, Central Africa, Australia, and England. In some studies, Chlamydia pneumoniae was recognized as a common cause in persons with early HIV disease, whereas Pseudomonas aeruginosa is recognized as a community- and hospital-acquired lower respiratory tract pathogen in patients with severe immunosuppression. Although antimicrobial therapy is frequently empiric, it should be tailored to the severity of illness, local prevalence of infections, resistance patterns, or when an etiologic agent is identified. The treatment response is similar in patients with and without HIV infection, but bacterial pneumonia may accelerate the progression of HIV disease. Preventative measures include use of the polyvalent pneumococcal vaccine, especially early in the course of HIV infection, when it is most likely to be effective. The incidence of bacterial pneumonia is also reduced in HIV-seropositive persons who use trimethoprim-sulfamethoxazole to prevent Pneumocystis carinii pneumonia.