Infectious complications are the cause of death in the overwhelming majority of patients with the acquired immunodeficiency syndrome (AIDS). These infectious complications are outstanding in severity and number. Infecting organisms are mainly those which take advantage of defective T-cell function. Many cause latent infections which can reactivate and cause disease in AIDS patients because of immunosuppression. Serological diagnosis of infections in AIDS patients is complicated because of the defect in B-cell function; rising antibody titers are not observed in most patients with infectious complications. The natural history of AIDS is one of repeated infections leading to a fatal outcome. However, treatment for most of the bacterial, fungal, and parasitic infections is effective. Treatment of some of these infections must be prolonged indefinitely because of the tendency to relapse.