Patients with HIV infection are at increased risk for community-acquired bacterial pneumonias, due in part to their defects in B-cell function. Streptococcus pneumoniae is the commonest cause of community-acquired pneumonia, with the second most common bacterial agent being Haemophilus influenzae. These two organisms account for about two-thirds of community-acquired bacterial pneumonias. Frequently bacterial pneumonias appear difficult to distinguish from Pneumocystis carinii pneumonia or other opportunistic lung infections, because of their atypical clinical and radiologic presentations. Community-acquired pneumonias may be recurrent but have low fatality rates. In comparison, nosocomial pneumonias occur primarily in patients with AIDS and are usually due to Staphylococcus aureus, Pseudomonas aeruginosa and other aerobic gram-negative bacilli. Nosocomial pneumonias have high fatality rates. S.aureus is an important cause of morbidity and mortality in patients with AIDS and has emerged as a secondary opportunist in lungs of patients with opportunistic diseases. While appropriate laboratory study is being done, empiric antibiotic therapy should be directed against the microorganisms above described.