IT has been reported that thyroid dysfunction is frequently associated with menstrual disturbances. Amenorrhea was one of the earlier known clinical changes associated with hyperthyroidism, as reported by von Basedow in 1840 [1]. Since then, amenorrhea has frequently been reported, along with a number of other changes in the menstrual cycle, including oligomenorrhea, hypomenorrhea and anovulation. These disturbances have been reported in more than half the patients with hyperthyroidism [2,3].On the other hands, hypermenorrhea and polymenorrhea have also been reported in about 50~80 % of patients with hypothyroidism [2-4]. These changes are sometimes related to ovulatory disturbance and subsequent infertility [5,6] and thus clinicians have focused on these problems for long time.However, these previous studies were performed in the 1950s and 1960s in a small number of patients with thyroid dysfunction. Abstract. The prevalence of menstrual disturbances, including secondary amenorrhea, hypomenorrhea, oligomenorrhea, hypermenorrhea, polymenorrhea and irregular menstrual cycle were prospectively examined in 586 patients with hyperthyroidism due to Graves' disease, 111 with hypothyroidism, 558 with euthyroid chronic thyroiditis, 202 with painless thyroiditis and 595 with thyroid tumor. In the overall patient group, the prevalence did not different from that in 105 healthy controls. However, patients with severe hyperthyroidism showed a higher prevalence of secondary amenorrhea (2.5%) and hypomenorrhea (3.7%) than those (0.2% and 0.9%, respectively) with mild or moderate hyperthyroidism. Moreover, patients with severe hypothyroidism had a higher prevalence (34.8%) of menstrual disturbances than mildmoderate cases (10.2%). Menstrual disturbances in thyroid dysfunction were less frequent than previously thought.