The serum levels of tumour marker CA 125 were measured in 162 patients with various digestive tract malignancies and in 155 patients with benign digestive tract diseases. The highest frequency of elevated CA 125 values (greater than 35 U ml-1) was found in patients with liver cancer (78%), but the level was equally often elevated in liver cirrhosis (78%). Two-thirds of the patients with biliary tract cancer had an increased CA 125 concentration, while four patients with benign biliary diseases had an elevated value. The serum level of CA 125 was elevated in only 20% of 60 patients with primary colorectal cancer, and in none of those with local disease (Dukes A or B). The CA 125 concentration seldom increased in patients with recurrent colorectal carcinoma. Twenty-three per cent of 44 patients with gastric cancer had an elevated CA 125 value. Two of 33 patients with benign colorectal and one of 68 patients with benign gastric diseases had an increased CA 125 concentration. The serum values of CA 125 showed no correlation with those of tumour markers alphafetoprotein (AFP), carcinoembryonic antigen (CEA) or CA 19-9. AFP was superior to the other markers in the diagnosis of liver diseases, while CA 19-9 showed the greatest accuracy in gastric diseases. In colorectal diseases, CEA had a higher sensitivity, but a lower specificity than CA 125 and CA 19-9. CA 125 and CA 19-9 had similar sensitivities for biliary tract cancer.(ABSTRACT TRUNCATED AT 250 WORDS)