A sudden increase in the US incidence of Kaposi's sarcoma in the past two years proved to be due to clinically very malignant cases, occurring nearly exclusively in young homosexual men. In the same population, lethal infections with Pneumocystis carinii occurred and also other infections usually associated with immunologic deficiency. is a working It is a working hypothesis that repeated infections with cytomegalovirus or use of the drug amyl nitrite, which is common among homosexuals, may cause immune depression with Kaposi's sarcoma and infection with usually non-pathogens as a secondary effect. An ongoing study of the life-style of Danish homosexuals should determine if factors incriminated by the US studies are present in both countries.