A 69-year-old man on hemodialysis for chronic renal failure was diagnosed with ascending colon cancer, and received surgical resection. Multiple liver metastases were detected after surgery. He was administered modified FOLFOX6 therapy (reducing the dose to 50%), and showed severe disturbance of consciousness due to hyperammonemia on treatment day 6. After treatment with daily hemodialysis, branched-chain amino acid solutions, lactulose and rifaximin, his conscious level improved on day 9. Intensive chemotherapy in dialysis patients should be carefully performed considering the serious adverse events including hyperammonemia.