Pulmonary infections are a very common complication in acquired immune deficiency syndrome (AIDS) patients. These infections may be severe enough to initiate the admission of these patients to intensive care units (ICU). Pneumocystis carinii pneumonia (PCP) is the most frequent cause of ICU admission because of acute respiratory failure. Mortality of ICU-admitted patients with this infection has changed with time. Initial reports confirmed a high mortality (80% to 90%). After 1985, the mortality rate decreased (50%). Factors such as the use of corticosteroids, better patient care, and a better knowledge of the disease probably explain this change. In recent years (1990 to 1995), mortality has worsened again, perhaps, because ICU facilities were offered more liberally to patients failing aggressive conventional treatment, including adjuvant therapy with corticosteroids. However, for those patients able to be discharged, the prognosis is not worse than expected according to the stage of their human immunodeficiency virus-1 (HIV-1) infection and immunologic status. Consequently, at least a limited period of ICU care and some respiratory support (either continuous positive airway pressure or mechanical ventilation) should be considered and offered to all HIV-1-infected patients with PCP and respiratory failure. Cytomegalovirus may be another cause of severe pulmonary infection in AIDS patients. This infection is difficult to diagnose; hence, it should be suspected when patients with PCP do not progress appropriately, or when no responsible pulmonary pathogen is found. When associated with PCP, mortality is very high. Disseminated tuberculosis is another potential cause of severe respiratory failure and respiratory secretions should be routinely examined for acid-fast bacilli in AIDS patients with pulmonary infiltrates. Finally, bacterial pneumonia (Streptococcus pneumoniae, Neisseria catarrhalis, Haemophilus influenzae, Staphylococcus aureus, and Pseudomonas aeruginosa) may also be the etiological agents of severe acute respiratory failure. Empiric antibacterial treatment to cover these microorganisms should be given when a bacterial agent is suspected.