Infusion of 5'-dFUrd (2.0-3.0 g/m2 over 1 h on days 1-5 every 3rd week) resulted in one partial response in 21 patients with advanced and progressing colorectal cancer. No patient had received chemotherapy before the 5'-dFUrd trial. Hematological and gastrointestinal toxicity were generally mild. In 4 patients peripheral neurotoxicity was diagnosed during treatment, whereas transient cerebellopathy was observed in one. Cardiac side effects (repeated angina pectoris following 5'-dFUrd infusion) led to discontinuation of treatment after two courses in one patient. It is concluded that 5'-dFUrd at the above doses is not superior to conventional 5-FU treatment in colorectal cancer. Neurological and cardiac side effects are rare but may be a problem in individual patients.