Prolonged exposure to 5-fluorouracil (5-FU) is effective for gastrointestinal malignancy (GIM) and it is considered synergistic or additive to concurrent radiotherapy. Oral 5-FU (OF) could represent an easy therapy. The present study prospectively tested the toxicity and effectiveness of OF in GIM by means of 5-FU mannitol-coated tablets (MCT) at 275 or 225 mg/m2/day according to the patients age (65 years cut-off) for a period of 4 weeks every 7 weeks. Also the drug given over 5 days a week for 4 weeks was studied to assess OF toxicity over a time corresponding to that used in standard radiotherapy. Quality of life (LQ) was analyzed. Patients were 27 individuals (20 males), aged 43-70 years, pretreated with radiotherapy (four patients) or i.v. 5-FU-based chemotherapy (five patients), and with progressive malignancy of colorectum (six patients), stomach (five patients), pancreas (four patients) and liver (two patients). The total number of cycles was 91 and 16 patients had more than two cycles. Myelotoxicity was consistently absent; other toxicities greater than WHO grade 1 were: nausea (grade 2 in four patients), diarrhea (grade 2 in six and grade 3 in 11), palmar erithema (grade 2 in one), brown-turning skin (grade 2 in one) and CNS (grade 2 in one). Diarrhea was less frequent (p = 0.007) in gastric and in colorectal than in pancreas and liver cancer patients. In the 10 patients given the drug of 5 days a week, diarrhea was practically absent. LQ was above 90%. Fourteen patients (51%) had total arrest of disease, and 2 among 16 colorectal cancer patients had PR (12.5%). In conclusion, the MCT-OF was tolerated and as effective as the classic i.v. 5-FU at nonmyelosuppressive dose. The MCT-OF dose recommended for further studies is 275 mg/m2/day (or 225 above 65 years) for 4 weeks followed by a 2 week rest period.