Irinotecan has proven anti-tumor activity as induction treatment in combination with 5-fluorouracil (5-FU) or as second-line treatment after 5-FU in patients with metastatic colorectal cancer. The aim of the present phase II study was to evaluate irinotecan as third-line chemotherapy in patients with colorectal cancer after sequential treatment with bolus 5-FU followed by an infusional 5-FU regimen. Patients pretreated with bolus 5-FU/folinic acid and the infusional 5-FU/folinic acid regimen were treated with 350 mg/m irinotecan i.v. once every 3 weeks in a multicenter phase II study. Tumor size was measured every cycle and treatment with irinotecan was continued until the occurrence of progressive disease or unacceptable toxicity. A total of 50 pretreated patients were included. Of the 45 evaluable patients, 13.3% [n=6, 95% confidence interval (CI) 5.1-26.8] attained a response (complete/partial response) to treatment lasting 5.6 months (95% CI 4.2-6.3) and in four patients response has been confirmed (8.9%, 95% CI 2.5-21.2). Disease stabilization was noted in 51.1% of the patients (n=23, 95% CI 35.8-66.3). The median duration of response/disease stabilization was 4.2 months (95% CI 3.2-6.0). Median overall survival was 7.9 months (95% CI 6.1-11.1), corresponding to a calculated 1-year survival of 28.3% (95% CI 15.2-41.3). Severe neutropenia occurred in 14% (n=7) and anemia grade III in 6% of the patients (n=3). The most frequent non-hematological toxicity grade III/IV related to treatment was diarrhea in 24% of the patients (n=12), followed by vomiting in 8% (n=4) and constipation as well as infection in two patients each (4%) (evaluable n=50). We conclude single-agent irinotecan is an effective and well-tolerable treatment in pretreated patients with metastatic colorectal cancer after failure of bolus and infusional 5-FU/folinic acid regimens. Elderly patients had the same probability to respond.