This study was undertaken to determine the prevalence of radiologically detectable pituitary tumors among patients seen initially in a gynecology clinic for a complaint of secondary amenorrhea. In a group of 144 women with secondary amenorrhea of more than 6 months' duration, 13 had radiologic abnormalities of the sella turcica detectable with standard skull films without tomography. These gynecologic patients composed more than half of the women in the reproductive age group who were diagnosed at this medical center as having a pituitary tumor. Only one patient with oligomenorrhea or secondary amenorrhea of less than 2 years' duration had a detectable tumor. Thus, for patients with short-term amenorrhea, the risk of having a pituitary tumor is small, especially in the absence of galactorrhea, headache, or changes in vision. However, 12 of 71 patients with amenorrhea longer than 2 years (17%) had detectable tumors and the risk appears to increase progressively with time. Assay of adrenal and thyroid hormones did not discriminate between patients with and without pituitary tumors. Early detection of these tumors is aided by serial sella x-rays, tomograms, prolactin assays, and newer pituitary function tests. Patients presenting with postpill amenorrhea and postpartum amenorrhea illustrate the hazard of interpreting a temporal relationship as a causal one.