The adenocarcinoid tumor is a potentially malignant neoplasm usually arising in the appendix. We describe a 55-year-old woman with a ureteroileal conduit in place for 12 years who suffered obstruction of the ileal segment of the conduit, with recurrent pyelonephritis and abdominal pain. Pathological examination of a segmentally resected portion of the conduit revealed a submucosal proliferation of nests of argentaffin/argyrophil cells and large, mucin-distended goblet cells. The patient showed no evidence of metastasis and has done well to date following excision. A discussion of neoplasms arising in the ureteroileal conduit is included.