In the next decade, longer survival of patients with cancer and more-aggressive therapies applied to common conditions, such as asthma and rheumatoid arthritis, will result in a larger population with significant immune system defects. Many in this population will be at risk for opportunistic infections, which are familiar to doctors who have treated people infected with human immunodeficiency virus (HIV). However, the epidemiology, presentation, and outcome of these infections in patients with an immune system defect, other than that caused by HIV infection, may be different than those encountered in patients with acquired immunodeficiency syndrome. Reviewed are 4 common opportunistic infections: Pneumocystis carinii pneumonia, cryptococcosis, atypical mycobacterial infection, and cytomegalovirus infection. Emphasized are the important differences among these groups at risk.