Thirty-two patients with metastatic colorectal cancer were entered in a phase II study of ACNU as first-line treatment. ACNU was given at a dose of 100 mg/m2 i.v. every six weeks. In 30 evaluable patients we observed partial responses lasting 5 and 8 months in 2 patients with liver metastases, and stable disease in 4 patients. The toxicity mainly consisted of leuco- and trombocytopenia. We conclude that in this dose and schedule ACNU has only very limited activity in metastatic colorectal cancer.