Doxifluridine (5-dFUR) is a fluoropyrimidine derivative, which is preferentially converted to 5-fluorouracil (5-FU) within tumour tissues. Although the activity of 5-FU in metastatic colorectal cancer is well recognised, resistance to this agent is frequently observed and remains its major limitation. The aim of this phase II study was to evaluate the activity of oral and i.v. 5-dFUR in metastatic or locally advanced colorectal cancer patients, who had been previously treated with a 5-FU containing regimen in either an adjuvant or metastatic setting. We treated 48 patients who, on the basis of tumour progression during, or within 8 weeks of the discontinuation of 5-FU therapy, were considered 5-FU resistant, 14 of the patients received 5-dFUR 3000 mg/m2 as a 1-h i.v. infusion, combined with L-leucovorin 25 mg/dose on days 1-5, every 3 weeks; the remaining 34 received oral 5-dFUR 1200 mg/m2 for 5 days followed by 5 days off. Oral L-leucovorin 25 mg/dose was administered 2 h before 5-dFUR. On the basis of WHO criteria, 4/14 (29%, 95% CI 4-51) partial responses were noted in the i.v. treated patients, and 4/34 (12%, 95% CI 1-23) in those treated orally. The radiological examinations documenting the response were a CT scan in 4 cases, ultrasound in 2 and NMR in 2. The median response duration was 6 months (range 3-11+), whereas the median time to treatment failure was 4 months (range 2-17). The responses were achieved in cases previously treated with a median of 9250 mg/m2 (range 5500-18,650) of 5-FU. No CTC-NC1 grade 4 toxicity was observed, although grade 3 diarrhoea occurred in 5 of the orally treated and in 3 of the intravenously treated patients. This is the first report documenting the efficacy of 5-dFUR in patients resistant to 5-FU therapy, and suggests that there is an absence of complete cross-resistance between these two fluoropyrimidines.