The ultrastructural features of three ovarian sex cord tumors with annular tubules were analyzed. The cells had deeply indented nuclei, interdigitating plasma membranes joined by abundant desmosomes, and numerous randomly distributed microfilaments that often complexed with desmosomes. Charcot-Bottchner crystalloids were absent. None of the tubular structures had true lumens. The characteristic central hyaline bodies resembled some forms of Call-Exner bodies and were composed of concentric layers of basal lamina, which often were continuous with redundant basal lamina of the peripheral basement membrane. Because of the clinical, histologic, and ultrastructural similarities to granulosa cell tumors and normal preovulatory granulosa cells, classification of the sex cord tumor with annular tubules is proposed as a distinctive annular and membranous variant of granulosa cell tumor.