Nine of 12 girls diagnosed consecutively as having severe and longstanding primary hypothyroidism were found to have multicystic ovaries when evaluated by pelvic ultrasound examination. The cysts resolved rapidly with thyroid replacement therapy. The patients had either elevated or high normal plasma luteinizing hormone levels which decreased markedly after thyroid therapy. Five of seven patients tested with luteinizing hormone-releasing hormone had attenuated gonadotropin responses; the other two reacted normally. The pathogenesis of the cysts remains uncertain. Use of pelvic ultrasound scan has documented a high frequency of multicystic ovaries in girls with primary hypothyroidism.