Objective: To report a case of ovarian cyst formation and myxedematous infiltration of the ovary in a subject with primary hypothyroidism. Design: Retrospective case report. Setting: University hospital. Patient(s): A 16-year-old female adolescent with pelvic pain, galactorrhea, irregular menses, and ovarian cysts on pelvic examination. Intervention(s): Laparotomy with bilateral ovarian wedge resection and thyroid replacement therapy. Main Outcome Measure(s): Ovarian histopathology, thyroid function tests, and menstrual history. Result(s): Resolution of patient's pain, galactorrhea, and resumption of normal menses. Conclusion(s): Ovarian cyst formation may accompany primary hypothyroidism in the child with accelerated or delayed sexual maturation. To date, the underlying pathophysiology of the morphological changes in the ovary remain enigmatic. This case report provides the first insight into the actual histologic changes that occur in ovaries of subjects with primary hypothyroidism without secondary ovarian pathology such as torsion. There is clear evidence of myxedematous infiltration into the ovarian stroma without luteinization of the theca interna. These microscopic findings suggest that local changes occurring independent of gonadotropin stimulation may contribute significantly to altered morphology of the ovaries in primary hypothyroidism.