Cell-mediated immune responsiveness was evaluated in 8 patients with dilated cardiomyopathy (CDM). The percentage of blood OKM1 (monocytes), OKT3 (T-cells), OKT4 (helper cells), OKT8 (suppressor cells) and Leu7 (NK) positive cells remained within the normal range whereas the percentage of Leu11 (NK) and OKIa1 (Ia determinant) positive cells was found to be decreased. The natural killer (NK) cell cytotoxicity was significantly lowered. The ConA-suppressor cell activity was seen to be enhanced while the responsiveness of lymphocytes to ConA was diminished. Thus, no consistent numerical changes in lymphocyte subsets were observed in patients with CDM. Some functional deficiencies were, however, detected, and it is possible that these may play a role in the pathogenesis of CDM.