Systemic chemotherapy based on 5-fluorouracil (5-FU) is a standard treatment for unresectable or recurrent large intestinal cancer. Although hyperammonemia is a known side effect of 5-FU that can cause serious pathological conditions, only a few cases have been reported. We describe 4 cases of 5-FU-related hyperammonemia with impairment of consciousness in patients who received 5-FU chemotherapy for large intestinal cancer with multiple liver metastases. Hemodialysis was effective in 1 severe case. There have been no detailed reports on the use of hemodialysis for hyperammonemia caused by 5-FU. Renal dysfunction is considered to be a risk factor for hyperammonemia caused by 5-FU and it is necessary to pay particular attention in patients with renal dysfunction who receive chemotherapy with 5-FU. Here we summarize our cases together with 16 previously reported cases of hyperammonemia caused by 5-FU in Japan.