Fifty patients were randomized to receive adjuvant intraperitoneal 5-fluorouracil (5-FU, 500 mg/m2/day) and intravenous leucovorin (60 mg/m2/day) and 51 to receive placebo after curative surgery for colorectal cancer. Treatment started on the day after surgery and continued for 6 days. One case of stomatitis, one of leucopenia and one case of abnormal liver function tests were the only chemotherapy-related toxic effects. From the second day of treatment, pain during intraperitoneal infusions occurred more frequently in the 5-FU group, although statistical significance was only attained on day 2 (P < 0.05). The groups did not differ substantially regarding any other adverse effects, the incidence of surgical complications, second laparotomies, time from surgery to discharge, or premature treatment terminations. The postoperative course after intraperitoneal 5-FU and intravenous leucovorin was thus not more complicated than that in patients treated with placebo. The tolerance was acceptable and chemotherapy-related toxicity was rare. Thus important prerequisites exist for more widespread use of the present regimen in order to evaluate its impact on survival.