The microorganisms that regularly infect patients with the acquired immunodeficiency syndrome (AIDS) have become well recognized. Most take advantage of defects in T-lymphocyte function, but others, such as Streptococcus pneumoniae and Haemophilus influenzae, take advantage of B-cell defects. Still others, such as Staphylococcus aureus and Shigella species, occur or persist for reasons that are unclear. Infections with organisms associated with hospitalization and medical procedures are also seen and should be anticipated. Among the infections taking advantage of T-cell defects, Pneumocystis carinii pneumonia is the most commonly diagnosed, but cytomegalovirus infection may be equally common. Disseminated Myobacterium avium-intracellulare infection has been found in one half of our patients at postmortem examination. The retrovirus responsible for AIDS commonly infects the central nervous system, as does Toxoplasma gondii. Although candida infections are common, dissemination is uncommon. Many of the infections respond to appropriate therapy but tend to recur when treatment is stopped. Often treatment courses must be prolonged even beyond those used in other immunocompromised hosts.