Indirect data supporting a preexisting immunologic impairment in patients with Hodgkin's disease (HD) have been presented in recent years. These immunologic defects are supposed to be related to genetic and/or environmental factors. In this study, 65 first-degree relatives and 12 spouses of 21 consecutive patients with HD were studied immunologically. Furthermore, seven twin pairs in which one partner had HD and four additional nonmatched healthy co-twins were also included in the study. A decreased lymphocyte DNA synthesis induced by Concanavalin A, a high spontaneous DNA synthesis, or a low CD4+/8+ ratio was found in 21 (32%) consanguineous, two (17%) nonconsanguineous relatives, and five (50%) healthy co-twins. The corresponding figures for the untreated patients with HD and the control series were 14 of 21 (65%) and 21 of 127 (16%), respectively. Total lymphocyte counts or lymphocyte subpopulations did not differ between HD relatives and controls. The increased frequency of blood lymphocyte defects among consanguineous first-degree relatives favors the existence of a genetically determined immune deficiency in at least a proportion of apparently healthy relatives of patients with HD. However, nongenetic factors such as age and environment may add to the defect.