As a diagnostic aid of malignant hepato-biliary tract disease, carcinoembryonic antigen (CEA) levels in the bile and serum were evaluated in 12 patients with benign and 19 patients with malignant hepato-biliary diseases. Of the 12 patients with benign disease, 3 had a residual choledocholithiasis. CEA levels were determined in 7 patients with cancer of the head of pancreas or of the duodenal ampulla. Bile samples were obtained via biliary tract drainage after allowing for sufficient time to exclude the effects of pre-existing bile stasis or inflammation. The average serum CEA levels from 8 patients with benign disease were 1.5 +/- 0.23 ng/ml in contrast to 3.3 +/- 0.55 ng/ml in 18 with a malignancy (p less than 0.05). The average CEA levels in bile from 9 patients with benign and 19 with a malignancy were 1.7 +/- 0.31 ng/ml and 7.6 +/- 1.70 ng/ml respectively (p less than 0.01). In 3 with residual choledocholithiasis, serum and bile CEA levels were 2.0 +/- 0.46 ng/ml and 13.1 +/- 6.47 ng/ml. The serum and bile CEA levels from 7 patients with cancer of the head of the pancreas or of duodenal ampulla were 2.5 +/- 0.32 ng/ml and 8.8 +/- 3.3 ng/ml, respectively. Although measurement of both serum and bile CEA levels in patients with hepato-biliary tract disease proved to be useful for differentiation of malignant from benign disease, the high value obtained strongly suggests the presence of a malignancy in addition to the residual choledocholithiasis and cancer of the head of the pancreas or of the duodenal ampulla.