Fifty-three examples of developmental "tailgut cysts" in the retrorectal space occurred predominantly in women and caused symptoms of mass effect or pain in 51%. The lesions were usually multicystic and lined by a variety of epithelial types, including ciliated columnar, mucin-secreting columnar, transitional, and squamous epithelium. Inflammation was present in 50%. In one patient a poorly differentiated adenocarcinoma was present. These cysts are most likely derived from remnants of the embryonic tailgut and differ from teratomas. Complete excision of the multilocular and multicystic process prevents recurrent draining sinuses and eliminates the possibility of malignant change.