The T lymphocyte function in 59 patients with malignant biliary obstruction undergoing pre-operative endoscopic drainage (group Ia, n = 24) or surgery (group Ib, n = 35) was evaluated by mitogen stimulation test with phytohaemagglutinin. The T lymphocyte function before endoscopic or surgical intervention was found to be impaired as compared with patients with gastric cancer (group II, n = 27) and with normal persons (group III, n = 19). Regression analysis showed a significant negative correlation between T lymphocyte function and the serum bilirubin level (correlation coefficient -0.3, P = 0.01) and a positive correlation with serum albumin level (correlation coefficient 0.34, P = 0.01) and serum transferrin level (correlation coefficient 0.45, P = 0.001). After 18 +/- 3 days of endoscopic biliary drainage, the T lymphocyte function of group Ia patients did not change substantially. At postoperative day 14, there were more patients in both groups Ia and Ib having deterioration of T lymphocyte function than those with improvement. The incidence of postoperative sepsis was found to be significantly higher in patients with deterioration than those with improvement of T lymphocyte function (18/31 vs 7/26, P = 0.036). It is concluded that endoscopic biliary drainage and surgery could not reverse the T lymphocyte dysfunction in patients with malignant biliary obstruction.