T otal lymphocyte counts, and the percentage of T and B lymphocytes and monocytes in untreated patients with Hodgkin's disease were not significantly different from those observed in normal donors. At the completion of radiotherapy, the mean total lymphocyte count of 503/mm3 was 4 SD below the mean for normal controls. Although a group of 26 patients in continuous complete remission from 12 to 111 mo after radiation treatment regained normal total numbers of lymphocytes and monocytes, they exhibited a striking T lymphocytopenia and B lymphocytosis. Concomitantly, there was a significant increase of null (neither T nor B) lymphocytes. The response of peripheral blood lymphocytes to phytohemagglutinin, concanavalin A, and tetanus toxoid before treatment was significantly impaired. 1-10 yr after completion of treatment there seemed to be little or no recovery of these responses. The capacity of peripheral blood lymphocytes to respond to allo-antigens on foreign lymphocytes in vitro (mixed lymphocyte reaction) was normal in nine untreated patients. However, the mixed lymphocyte reaction was markedly impaired during the first 2 yr after treatment. There was a partial and progressive restoration of the mixed lymphocyte reaction during the next 3 yr, and normal responses were observed in patients in continuous complete remission for 5 yr or more. The in vivo response to dinitrochlorobenzene was also examined. 88% (15/17) of patients initially sensitive to dinitrochlorobenzene were anergic to the allergen at the completion of a course of radiotherapy, but nine of these regained their hypersensitivity response during the 1st yr after treatment. This data suggests that there is a sustained alteration in both the number and function of circulating T cells after radiation therapy in patients with Hodgkin's disease which may persist for as long as 10 yr after treatment. The restoration of cell mediated immune functions after radiotherapy is time dependent and its kinetics may differ for various T-cell functions. The implications of these findings with respect to the state of immunological competence after radiotherapy are discussed.