The distribution of T-lymphocyte subsets was studied in patients with myocardial infarction, peripheral arterial disease and thrombophlebitis. The percentage of SRBC-rosette forming active - E[A] and total - E[T] lymphocytes in patients and healthy controls was not different. A significant increase in the percentage and number of theophylline-resistant - E[Thr] and T[M] - helper cells was found in patients with myocardial infarction or with thrombophlebitis, and a moderate elevation was observed in patients with peripheral arterial disease. No difference between patients and controls could be shown in the percentage and number of theophylline-sensitive E[Ths] and T[G] - suppressor cells. The imbalance of T-lymphocyte subsets is thought to be the consequence of sensitization against vascular tissues during vascular diseases.