Irinotecan (CPT-11) is an effective drug in patients with advanced colorectal cancer (CRC). Little is known about its efficacy and safety in previously treated patients with poor performance status. We prospectively evaluated the antitumor efficacy and safety of CPT-11 monotherapy in this setting. Thirty-four patients with poor performance status (Karnofsky score between 60 and 80) and/or progressing on one or more previous 5-FU-based chemotherapy lines for advanced colorectal adenocarcinoma were enrolled in this study. Treatment consisted of irinotecan (CPT-11) at 100 mg/m(2) administered as a 60-min iv infusion every week for four consecutive weeks followed by a 2-wk rest period until disease progression or unacceptable toxicity. The overall objective response rate (WHO criteria) for the 34 patients included was 20.6% [95% confidence interval (CI): 6.3%-34.9%]. Stable disease was obtained in 13 patients (38.2%) and 14 patients (41.2%) progressed. The median time to disease progression was 5.5 mo (range: 0.9-17.5) and the median survival was 8.3 mo (95% CI: 1.7-16.9). Overall, weekly CPT-11 was well tolerated with grade 3/4 neutropenia as the main hematological toxicity (11 patients: 32.4%; 14 infusions: 3.3%), and delayed diarrhea (10 patients: 29.4%; 16 infusions: 3.8%) as the main grade 3/4 non-hematological toxicity. In conclusion, weekly CPT-11 at 100 mg/m(2) for four consecutive weeks followed by a 2-wk rest period showed antitumor efficacy and may be safely administered to heavily pretreated patients with advanced colorectal cancer and a poor performance status. Weekly CPT-11 monotherapy may be considered as a therapeutic option for this population of patients.