Patients with advanced colorectal cancer were randomized to receive either fluorouracil (5-FU) 370 mg/m2 IV days 1 to 5 followed by weekly applications of 5-FU 600 mg/m2 or the same doses of 5-FU preceded by folinic acid 200 mg/m2. Because of toxicity, the weekly 5-FU dose in the combination treatment schedule was reduced to 500 mg/m2 in the course of the study. As of November 1990, 135 patients entered the study; 71 have received combination therapy, and 64 monotherapy. Sixty-three and 59 patients, respectively, are included in the present interim analysis. The two groups are well matched for age, performance status, site of disease, number of metastatic sites, and biochemical parameters. Treatment results are evaluable in 118 patients. Thirty percent receiving combination treatment and 20% receiving monotherapy achieved a complete or partial remission. There is no survival time difference between the groups. However, time to progression is superior in the combination treatment group (median 26 weeks compared with 13 weeks). The main toxicity was diarrhea during the weekly therapy. This was especially true for patients receiving combination treatment before the reduction of 5-FU dosage. In contrast to only four of 56 patients with monotherapy, 14 of 39 with the combination treatment at the initial dosage had severe diarrhea with two treatment-related deaths in this latter group. By reduction of 5-FU dosage during the weekly therapy severe diarrhea could be clearly reduced with only one of 18 patients suffering from diarrhea of World Health Organization grade 3. Other toxicity was usually mild. In conclusion, a prolongation of time to progression could be achieved by combination treatment of folinic acid and 5-FU, which was well tolerated when the weekly dose of 5-FU did not exceed 500 mg/m2.