To determine the immune disturbances involved in the pathogenesis of dilated cardiomyopathy (DCM), the conditions in humoral and cellular immunity of DCM patients were studied. To examine the status in humoral immunity in DCM patients, circulating anti-heart antibodies (AHAbs) were analyzed. Circulating-AHAbs were classified into 4 types "heterophile-like", "intracellular", "intercalated disc" and "nuclear" patterns by indirect immunofluorescence technique, and a high rate of heterophile-like antibody was found in DCM. The cytotoxicity of this antibody was examined in cultured myocardial cells using the two-step method of the complement dependent cytotoxic test. The mean cytotoxic index (CI) value of the heterophile-like antibody positive sera was 22.3, which showed an apparent cytotoxicity against the cultured cells and it may be complement dependent. In addition, the frequency of lymphocyte subsets using monoclonal antibodies (OKT4, OKT8, Leu7, Leu11) and natural killer (NK) activity were evaluated to determine whether DCM patients had an imbalance in cellular immune reactions, which support the hypothesis of an immune disturbance as the pathological mechanism of DCM. The peripheral lymphocyte counts were significantly lower in patients with DCM (1737 +/- 874/mm3) than in normal controls (NC, 2088 +/- 556/mm3) and in patients with ischemic heart diseases (IHD, 2395 +/- 469/mm3, both p less than 0.01). The percentage of T, B, OKT4 and OKT8 positive cells was not statistically different among DCM, IHD, and NC groups, whereas the percentage of T gamma cells was significantly reduced in DCM patients (6.5 +/- 5.0%, p less than 0.05). NK functional activity as tested in DCM patients was frequently deficient (24.1 +/- 16.7% in DCM, 36.7 +/- 12.2% in NC). After 4-hr incubation with recombinant interleukin2 (rIL2), rIL2 induced the enhancement of NK activity in 3 out of 4 DCM patients with low NK activity, although, there was a non-responder to rIL2 in this group. These results suggested that DCM patients have a low IL2 production and/or less numbers of mature cells with NK cell function. Therefore, an imbalance in humoral and cellular immune reactions may cause insidious myocardial damage and subsequently lead to development of DCM.