In a double blind study, patients with operable carcinoma of the colon and the upper rectum, who have undergone a macroscopically complete resection of their tumor, were randomized to receive either (i) polyadenylic-polyuridylic acid (AU), one i.v. injection of 60 mg (in 50 ml of solution) once a week for 6 weeks, or (ii) a placebo (P) one i.v. injection of 50 ml of a saline solution with the same schedule. From January 1983 to December 1986, 288 patients were enrolled: 145 in AU group and 143 in P group. The main clinical and pathological characteristics were equally distributed throughout the two groups. There was a significant difference (P0.02) in the overall survival (OS) between the two groups, in favor of the P group. The 5-year OS rate was 68% (SD = 4%) in the AU group versus 81% (SD = 3%) in the P group. Thus, AU as a single adjuvant, appears to be ineffective and therefore has no indication in the treatment of colorectal carcinoma.