Sepsis is a common and occasionally lethal complication of obstructive jaundice. The reasons for the increased susceptibility to infection are not fully understood. This study was conducted to examine lymphocyte subsets and natural killer cells of patients with obstructive jaundice in perioperative period. In these patients, when compared with normal controls, there was a significant reduction in the percentage of Leu 4 (pan T lymphocytes), Leu 3a (T helper cells) and Leu 7 (natural killer cells) before operation, and the immune suppression induced by surgical operation was more marked and persistent.