The sex cord tumor with annular tubules (SCTAT) is a distinctive ovarian neoplasm the predominant component of which has morphologic features intermediate between those of the granulosa cell tumor and those of the Sertoli cell tumor; focal differentiation into either granulosa cell or Sertoli cell tumor may occur. Of the 74 cases that form the basis of this investigation 27 were associated with the Peutz-Jeghers syndrome; these tumors were all benign and were typically multifocal, bilateral, very small or even microscopical in size and calcified. Twelve of the 27 patients had symptoms suggestive of hyperestrinism attributable to the SCTAT; menstrual irregularity had occurred in eleven cases and postmenopausal bleeding in one. Four of the 27 patients had "adenoma malignum" of the cervix and two of them died of it. The 47 tumors from patients without evidence of the Peutz-Jeghers syndrome were unilateral and usually large. Twenty-five of them were accompanied by symptoms suggestive of hyperestrinism, such as menstrual irregularity, postmenopausal bleeding or sexual precocity; seven were malignant and four of these were fatal.