Tumor-associated trypsin inhibitor (TATI) was assayed in serum of patients with gastroenterological diseases. Of the patients 92 had gastric cancer, 50 colonic cancer, 38 colitis, 36 polyposis of the colon, and 40 gastric ulcer. The cut-off level established on the basis of the mean concentration +3 SD of a reference population comprising 120 subjects was 32 micrograms/l. In gastric cancer TATI had a sensitivity and specificity similar to that of CA19-9, whereas its behavior in colon cancer was less satisfactory. Like other tumor markers TATI may be elevated in patients with inflammatory diseases. In our opinion TATI is a good tumor marker for gastric cancer and it is a useful complement to CEA and CA19-9.