Thirty-three consecutive patients with recurrent and/or metastatic colorectal carcinoma (CRC) refractory to previous chemotherapy have been treated with levofolinic acid (I-FA) 100 mg/m2 i.v. over 1 h infusion followed by 5-fluorouracil (5-FU) 600 mg/m2 i.v. bolus every week for 6 weeks followed by a 2 week interval. Patients also received rIL-2 s.c. at 3 MU daily from day 1 to day 5 of each week for at least four consecutive weeks per cycle. Enrolled patients were divided in two groups: (i) group 1 including patients with progressive tumor refractory to chemotherapy with I-FA + 5-FU given for metastatic disease and (ii) group 2 consisting of patients with diagnosis of metastatic disease within 3 months from the completion of adjuvant chemotherapy with 5-FU + levamisole (LEV) after primary surgery. No objective response was observed in the group of 11 patients with CRC resistant to previous I-FA + 5-FU, thus no further patient with progressive disease after I-FA + 5-FU was included in the trial. In the group of patients pretreated with 5-FU + LEV, four patients experienced a PR with a mean duration of 7.3 months (range > or = 4.0-8.6) for an overall response rate of 18% (95% CI 12-26%). A stabilization of disease was observed in five cases (23%) with a mean duration of > or = 5.6 months (range > or = 2.0-7.0). The remaining 13 patients progressed. No complete responses were achieved. The mean overall survival was > or = 9.5 months (range > or = 2.0-14.0). Toxicity was generally mild. This study demonstrates that the combination of s.c. rIL-2 and intravenous 5-FU + I-FA on a weekly schedule may be safely given to patients with metastatic CRC on an outpatients basis. The addition of low-dose rIL-2 does not modify the toxicity profile of 5-FU + I-FA, even if IL-2-related side-effects such as systemic symptoms or cardiac abnormalities are to be expected. The clinical activity of the combination is not good, at least in terms of response rate, even if the duration of partial responses may suggest to test rIL-2 in a prospective study with response duration and overall survival as the final end-points.