A woman with a history of thrombocytopenic purpura occurring approximately every twenty-eight days was studied extensively on the Metabolic Research Ward. The patient had been treated previously with splenectomy and hystero-oophorectomy without improvement. The cyclical nature of her thrombocytopenia was documented by repeated hematologic observations. Endocrinologic studies demonstrated no evidence of ovarian activity or cyclical variations in adrenal activity. Urinary gonadotropins (FSH) fell coincident with the fall in platelets and suggest that cyclical pituitary function might be related in some way. The exact etiologic mechanisms, however, were not disclosed by this study.