Immune function, T-lymphocyte subsets, serum quantitative immunoglobulin levels, serum lysozyme levels, and circulating immune complex levels were analyzed in patients with idiopathic dilated cardiomyopathy (IDCM). The percentage of helper/inducer T cells (OKT4) was higher and the percentage of suppressor/cytotoxic T cells (OKT8) was lower in IDCM patients than in healthy controls and in patients with ischemic heart disease. IDCM patients, in addition, have higher 5/9+ T cells, a T-cell subset known to give maximal helper activity in B-cell differentiation assays. Peripheral blood mononuclear cells (PBMC) from IDCM patients demonstrated a statistically greater ability to induce B-cell differentiation (helper T-cell function) into plasma cells and a hypofunctioning suppressor T-cell population in an in vitro pokeweed nitrogen (PWN)-driven B-cell differentiation assay. Serum immunoglobulin IgM levels were higher in IDCM patients, but serum lysozyme levels and serum immune complex levels in IDCM patients were normal. These data verify that an immunoregulatory defect exists in IDCM.