Abstract: Lymphocyte subsets have been measured in the blood of 31 patients with chronic aplastic anemia (CAA). The mean absolute value of helper/inducer T cells was decreased while that of suppressor/ cytotoxic T cells was increased. The mean Th/Ts ratio (1.07±0.11) was significantly decreased in the total population of CAA, in comparison to the mean ratio in normal population (2.06±0.08). A reversed Th/Ts ratio (＜1) was present in a large proportion (58%) of CAA, due to an absolute deficiency of helper/inducer T cells and an increase in suppressor/cytotoxic T cells. In the patients with CAA, a positive correlation existed between Hb and Th/Ts ratio (r=0.539; p＜0.01), although a correlation was found between Ts cells and neutrophils (r=-0.497; p＜0.01). Among five patients with CAA and a reversed Th/Ts ratio, four showed no growth of GM-CFUc colony, and in the seven patients with a normal Th/Ts ratio, only one had no colony formation. The majority of patients with CAA and a abnormal Th/Ts ratio did not respond to routine treatment, and had a more severe anemia. A normal distribution of lymphocyte subsets was present in four patients with CAA in remission. The results indicate that the lymphocyte subsets imbalance may be related to bone marrow failure in some patients with aplastic anemia.