Immunoregulation was assessed in a group of patients with myelodysplasia (MDS) by flow cytometric analysis of peripheral blood lymphocyte subsets and in vitro studies of mitogen-stimulated T-lymphocyte blastogenesis. Mitogenesis was significantly depressed in MDS patients compared to controls (p less than .001) and a similar defect was found in a small group of patients with untreated acute nonlymphocytic leukemia (ANLL) (p less than .005). The impaired mitogenic response ability of T-cells in these patients did not appear to be the result of alteration in lymphocyte subpopulation ratios. The observed defect might result from defective cooperation between T-lymphocytes and abnormal myeloid elements. Alternatively, the lymphocytes themselves could be derived from the abnormal clone and thus be functionally abnormal.