Sulindac, a nonsteroidal anti-inflammatory drug, causes regression of polyps in familial polyposis coli and may prevent new lesions. However, it is not clear whether the effect of sulindac in preventing polyps also applies to carcinoma. This case report describes a patient with familial polyposis coli who developed a carcinoma in a rectal segment after subtotal colectomy and ileorectal anastomosis. She had been treated with 450 mg sulindac daily for 28 months and was free of polyps during the latter 12 months of this period. However, despite intensive endoscopic follow-up, she developed an adenocarcinoma of the rectum. This finding may have important implications for our understanding of the development of colon cancer in familial polyposis coli and the use of sulindac to prevent it. Development of de novo carcinoma in microadenomatous tissue of the rectal mucosa, which bypasses the polyp-cancer sequence, must be considered as a possibility in these patients.