A study was performed to investigate a relationship between the status of T lymphocyte subsets in the peripheral blood and the pathogenesis of erythema nodosum leprosum (ENL) reactions. T lymphocyte subsets, B lymphocytes and serum immnnoglobulin levels were explored in 40 leprosy patients (10 tuberculoid (TT), 17 lepromatous (LL), and 13 lepromatous with ENL). In patients with LL, suppressor cells increased and helper cell diminished, resulting in a decrease in the helper/suppressor (Th/Ts) ratio to 0.86 + 0.20, as compared with controls (1. 77 + 0.41). But in ENL patients, there was a significant diminution of suppressor cells and an increase of helper cells, resulting in an increase in Th/Ts ratio to 2.08 + 0.30. Four weeks after treatment of ENL, Th/Ts ratio decreased to l.18+ 0.31 again. The mean percentages of B cells and serum immunoglobulin levels in with LL. and ENL increased significantly compared with controls. Only IgG increased in patients with ENL compared with patients with LL. The results in patiients with TT did not differ significantly from those of controls. The findings suggest that cell-mediated immune responses, imbalance in T lymphocyte subsets, may play a role in the pathogenesis of ENL, either directly or by inducing an antibody critical to the formation of the immune complex