Abstract: AObjective 　This study is to investigate the relationship between the UH₂-U ratio and gast rointestinal toxicity with 5-Fu chemotherapy, and prove evidence of theory in tumoral t reatment of individual. Methods 　70 patient s included 48 colorectal patients, 21 nasopharynx patient s and 1 esophageal patient . Endogenous uracil and dihydrouracil were assayed by HPLC ( High performance liquid chromatography) . The UH₂-U ratio ≥2. 05 was normal value. Results 　There were 53 patients whose UH₂-U ratios were normal. The grade Ⅰ～ Ⅱnausea/vomiting, diarrhea and mucosal respectively was 35, 26 and 17 patients. The patients whose UH₂-U ratio < 2. 05 respectively was 7, 7 and 9 patients. Grade Ⅲ～ Ⅳ occurred in 9, 9 and 7 patients. Which was significantly different ( P < 0. 05) . Diarrhea and mucosal were of statistic significance in different dosage and different infusion methods. Conclusion 　UH₂-U ratio can be used to predicate the toxicity of 5-Fu and to adjust the dose of 5-Fu or to change the therapy regimen before the treatment .