In a randomized study 52 patients with advanced colorectal cancer and measurable lesions were treated with doxifluridine 4000 mg/m2 or fluorouracil 450 mg/m2 i.v. on 5 consecutive days over 3 weeks. None had prior fluoropyrimidines except two who received adjuvant fluorouracil. Partial responses with a duration ranging from 259 to 406 days were observed in five patients treated with doxifluridine and two patients treated with fluorouracil. Toxic reactions were evaluated in 88 doxifluridine courses and 105 fluorouracil courses. The most frequent adverse effects were neurotoxicity (48% of patients) and mucositis (43%) for doxifluridine, leukopenia (48%) and nausea/emesis (37%) for fluorouracil. Mucositis, diarrhea, nausea, emesis and skin reactions were observed in both treatment groups. Fluorouracil produced neurotoxic effects in 26% of patients. Reversible cardiac dysfunctions were observed in four patients treated with doxifluridine, expressed by ectopic ventricular beats (2) precordial pains (1) and ventricular fibrillation (1). This latter toxicity justified the premature interruption of the study. Doxifluridine is an active agent in colorectal cancer. Compared to fluorouracil it produces, when used i.v., a lower myelosuppression and a greater incidence of neurological and cardiac toxicity.