This phase II trial studied the antitumor effect and toxicity of weekly irinotecan (CPT-11, 125 mg/m(2) 60 min iv infusion, weekly for 4 wk plus 2 wk rest) as second-line chemotherapy in patients with advanced colorectal cancer (CRC) resistant or refractory to prior 5-fluorouracil (5-FU) therapy. Sixty-nine patients with adenocarcinoma (57% in the colon and 43% in the rectum) were enrolled. The median number of treatment cycles received per patient was 4 (range, 1-6). Overall response rate was 18% (95% CI, 9-26), with 4 complete responses (6%) and 8 partial responses (12%), and a median duration of response of 8.1 mo (95% CI, 4.2-12.1). Stable disease was observed in 19 patients (28%). The median time to disease progression was 5.2 mo (95% CI, 4.3-6.1), and the median overall survival was 13.3 mo (95% CI, 9.8-16.8 months). The toxicity profile was favorable: grade 3/4 delayed diarrhea was observed in 10 patients (14.5%) in one cycle each, and grade 3/4 neutropenia in 6 patients (8.7%) and 6 cycles (3.3%). No febrile neutropenia or infection was documented. Grade 3/4 nausea and vomiting were reported in 1 (1.4%) and 7 patients (10.1%), respectively. In conclusion, this phase II trial showed a response rate and a toxicity profile of weekly CPT-11 in line with the results of prior phase II studies.