The data suggest that: The recent outbreak of KS and PCP is a single epidemic, that of immunosuppression among homosexual men and drug abusers. The public health significance of this epidemic is probably underestimated, and clinicians should be alert to more subtle indications of immunosuppression, such as nonfatal opportunistic infections, unexplained lymphadenopathy, or other tumors. Opportunities to clarify the relationships between the environment, immunology, cancer, and infections make this outbreak scientifically important. The sudden and highly focal occurrence of these illnesses among homosexual men and drug addicts suggests a potential for their prevention if risk factors or etiologic agent(s) can be identified. The staggering morbidity and mortality associated with this outbreak dictate immediate concerted efforts to identify risk factors.