Peripheral blood lymphocytes of diabetes mellitus patients were analyzed by flow-cytometry using monoclonal antibodies directed against cell surface markers present in T-and B-cells, monocytes and natural killer cells. The lymphocyte subsets were quantified and expressed in an absolute amount. The study included 17 patients with type I (insulin-dependent), 21 patients with type II (non-insulin-dependent) diabetes and 40 age-matched control subjects. Quantification of the cells present within different lymphocyte subsets revealed a general increase in both patient groups compared to their controls, with the exception of activated T-cells. However, no significant difference was found in the relative amount of T-helper cells and T-suppressor/cytotoxic cells of the diabetes patients when they were compared with their control groups. The fact that we found similar changes in lymphocyte subsets in both type I and type II diabetes suggests that the altered immunological state is secondary to the diabetes mellitus in general.