Carcinoembryonic antigen(CEA) levels were measured in the serum of 35 normal control subjects and 179 cases of various benign and malignant gastrointestinal diseases. Malignant gastrointestinal tumors include 69 cases of stomach cancer, 24 cases of hepatoma and 33 cases of colorectal cancer. Benign gastrointestinal diseases include 29 cases of peptic ulcer and 24 cases of liver cirrhosis. The results were as followings: 1) Mean serum CEA level in normal control subjects was 6.9¡¾3.3 ng/ml and there was no difference in mean serum CEA level between age and sex difference. 2) In malignant gastrointestinal tumors, mean serum CEA level in colorectal cancer, hepatma and stomach cancer, were 54.3¡¾88.9 ng/ml, 62.1¡¾99.7 ng/ml respectively. Serum CEA level showed positive rate of 67% in colorectal cancer, 63% in hepatoma and 625 in stomach cancer. There was no difference in mean levels and positivity of serum CEA between these 3 malignant tumor groups. 3) Positivity of serum CEA was 61% in malignant gastrointestinal tumor group in spite of 37% in benign gastrointestinal disease group. In both mean level and positivity of serum CEA, stomach cancer was much higher than peptic ulcer. But there was no difference in mean level and positivity of serum CEA level between hepatoma and liver cirrhosis. 4) In hepatoma serum CEA level showed positive rate of 62.5% and alpha-feto protein showed a rate of 58.3%. 5) Mean serum CEA levels in patients with cancer in rectal, cecal, sigmoid colon, ascending colon and descending colon were 73.7¡¾106.7 ng/ml, 69¡¾84.8 ng/ml, 15.7¡¾9.1 ng/ml, 7.5¡¾10.6 ng/ml and 4.0 ng/ml respectively. Positive rate of serum CEA showed 86% in sigmoid colon cancer, 68% in rectal cancer and 66% in cecal cancer. 6) In considering of histological background, there was no col1elation between the degree of differentiation of tumor cell and the serum CEA level in colorectal cancer. According to Duke's classification, the mean serum levels of CEA were 8.8¡¾11.4 ng/ml in group A, 15.3¡¾16.0 ng/ml in group B and 68.