Ulcerative colitis is a chronie inflammatog disorder of unknown cause, which primarily involves the eolonic mucosa. There is increased incidence of carcinoma in patients with uicerative colitis. The major independent factor for coloreetal cancer in patients with ulcerative colitis is duration of disease, extent of involvement, and onset of age. We report two cases of colon cancer which arose in patients with a long history of ulcerative colitis, A 52-year-old house wife was admitted due to bloody diarrhea. She had a 10 years history of ulcerative colitis with pancolitis. The pathologic specimen shows multicentric cancer in rectosigrnoid region and proximal ascending colon with histologic features of mucinous adenocarcinoma. She died of cancer recurrence in the rectum 13 months later. Seeond case is a 62-year-old woman who presented with diffuse abdominal pain. She had a 6 year history of ulcerative colitis with pancolitis. Right hemicolectomy was done. Pathologic specimens showed adenocarcinoma with crypt abscesses and mild dysplasia in the adjacent cecal mucosa. After chemotherapy, she is still alive without complication.