The subject matter of this volume, the basis for which was a conference held in Philadelphia which focused on the subject of infections, including their diagnosis and treatment, in immunocompromised individuals. The material is of particular importance today when placed against the background of the rapid spread of acquired immunodeficiency syndrome (AIDS). The first section dealt with the general subject of the immunocompromised host. Here, reviewed in detail, were the epidemiological and clinical aspects of opportunistic infections in patients with defective immune responses. It is widely acknowledged that infections are a major complication of the neoplastic process. Cancer-bearing patients are more prone to certain kinds of infectious and cancer chemotherapy almost always increases susceptibility to such infections. Depending upon the basic disease process of the cancer, a specific array of infectious diseases can be predicted. Patients altered in thymus-derived lymphocyte populations or mononuclear phagocyte capabilities resulting in defects in cell mediated immunity or delayed hypersensitivity become highly susceptible to certain groups of organisms, whereas, profoundly neutropenic patients usually become infected with different organisms. The types of infections noted are relatively predictable for the type of immune defect, with some variations according to epidemiological factors. Major advances have been made in the early diagnosis and treatment of infectious complications with increasingly effective antimicrobial agents and increasing knowledge of their use. The application of so-called preventive procedures has had limited value to date, including immunotherapy, which appears to hold much promise.