A 53-year-old man who had had an anal fistula for 20 years was admitted to our hospital with a large intestinal obstruction. Barium enema and colonoscopy confirmed advanced rectal cancer and we palpated a soft tumor, 3 cm in diameter, with inflammatory induration on the right side of the rectum. After draining a perianal abscess caused by the anal fistula, we performed low anterior resection. Histological examination of the perianal necrotic tissue obtained during resection of the perianal tumor encompassing the anal fistula revealed adenocarcinoma. Since the histology of the perianal lesion was identical to that of the rectal cancer, a diagnosis of cancer implantation rather than carcinoma originating in the anal fistula was entertained. Although the recurrence of rectal cancer by mucosal implantation is not uncommon, the coincidental implantation of rectal cancer in an anal fistula is extremely rare.