A total of 15 patients with measurable advanced colorectal adenocarcinoma were prospectively treated with fazarabine (Ara-AC), reconstituted in dimethyl sulfoxide, and administered at a starting dose of 48 mg/m2/day as a continuous intravenous infusion for three days. The dose was repeated every 21 days and dose escalations or reductions were made on the basis of toxicities encountered in the preceding course. No patient achieved either a complete or partial response. Major toxicities encountered were granulocytopenia, thrombocytopenia, nausea, vomiting, anemia, and headache. All toxicities were reversible upon discontinuation of the drug and no life-threatening toxicities occurred. These data indicate that further clinical trials in colorectal carcinoma with this agent and schedule of administration are not warranted.