We investigated the relevance of various factors concerning consciousness disorder accompanied by hyperammonaemia in 3 patients with advanced colon cancer when mFOLFOX6 was administered. (Subjects and Methods) In the 105 cases administered mFOLFOX6, blood ammonia, urea nitrogen, and creatinine levels were measured before and 24, 48 hours after the administration in the 47 cases. We also measured DPD in carcinoma by ELISA. (Results) Eight cases showed high levels of ammonia compared to before administration and 15 cases had higher levels after administration, but no clinical manifestation was detected. Consciousness disorder accompanied by hyperammonaemia occurred in 3 cases (2.9 %) after mFOLFOX6 was given several times. There was renal dysfunction in 2 cases. No correlation was found between DPD. (Results 2) There were some cases with high levels of ammonia who had no clinical manifestation. We concluded it is necessary to measure ammonia levels at the time of consciousness disorder on mFOLFOX6 used.