We report a unique case of rectal endometriosis mimicking solitary rectal ulcer syndrome. Several rectal biopsies were performed before the correct diagnosis of rectal endometriosis was made. The lesion had striking histologic features resembling colitis cystica profunda. The presence of two types of glands, i.e., colonic glands with marked depletion of mucin and endometrial glands, were readily apparent on immunohistochemical stain using anticarcinoembryonic antigen showing positive cytoplasmic stain for colonic glands but negative for endometrial glands. However, the distinction between colonic and endometrial glands was very difficult on hematoxylin-and-eosin-stained slides. Endometrial stroma was identified only in the sixth biopsy specimen. Although rare, rectal endometriosis should be considered in the differential diagnosis of solitary rectal ulcer syndrome.