A 27-year-old woman with a 9-year history of ulcerative colitis involving the entire colon was admitted to our hospital in August 1992 because of bloody stools and left lower abdominal pain. She had been treated with sulfasalazine since 1983 and the colitis had been clinically quiescent or mild for 7 years. She had also been diagnosed as having primary sclerosing cholangitis (PSC) 4 years prior to this admission, based on the clinical, laboratory, and cholangiographic findings. A barium enema and colonoscopy showed an irregular mass obstructing the bowel lumen in the distal portion of the descending colon. Biopsy specimens taken from the mass revealed moderately differentiated adenocarcinoma, and a subtotal colectomy was performed. Histologic examination of the mass lesion showed moderately differentiated adenocarcinoma invading the pericolic adipose tissue. She is currently alive 3 years after surgery. PSC has recently been reported as a risk factor for colonic neoplasia in patients with long-standing ulcerative colitis. In Japan, however, colorectal cancer associated with PSC and ulcerative colitis has rarely been reported. The present case suggests that the risk of colonic cancer is higher in patients with ulcerative colitis and PSC than in patients with ulcerative colitis alone.