A 55-year-old man with chronic hepatitis C had diarrhea with bloody stool in July, 2003 and ulcerative colitis was suspected. However, he quickly improved. He was treated with percutaneous radiofrequency ablation therapy for adenomatous hyperplasia in S5 of the liver in December, 2004. After the ablation therapy, he was treated with combination therapy of PEG-interferon alpha-2b and ribavirin for chronic hepatitis C. Because exacerbation of ulcerative colitis appeared 10 weeks after beginning of the treatment for hepatitis C, the combination therapy of PEG-interferon and ribavirin was discontinued. He was treated with mesalazine and steroid therapy for ulcerative colitis, and improved. We report the first case in Japan of the exacerbation of ulcerative colitis induced by the combination therapy of PEG-interferon and ribavirin for chronic hepatitis C.