A comprehensive review of the history of pregnancy gingivitis is presented. Histologic studies were made of the gingivae of a large group of pregnant women, of whom twenty are reported in table form. The biopsies were taken at different months during gestation from those women exhibiting clinical manifestations of pregnancy gingivitis. Clinical photographs were also taken and are included. Histologic studies were made of the gingivae of patients with endocrine disturbances and of monkeys treated experimentally with hormones. Photo-micrographs and clinical photographs of representative cases are included. A clinical classification of pregnancy gingivitis, representing the progressive changes in pregnancy, is presented. The most prominent changes in the gingivae and oral mucous membranes during pregnancy were a loss of surface keratin, hydropic alteration of the stratum spinosum, hyperplasia of the stratum germinativum, and inflammatory changes in the lamina propria. A comprehensive review of endocrine and metabolic relationships is presented. This includes a consideration of the action of the hypophysis, the thyroid gland, the adrenal cortex, the ovaries, the placenta, and the vitamins. The histologic changes apparently do not arise from a single etiological factor. Rather they are a manifestation of the altered nutritional and the metabolic status of the individual during pregnancy. However, the immediate cause of the gingival changes appears to be a diminished utilization of estrogen or a modification of estrogen metabolism. The mechanism involved may be explained in several ways: (1) Estrogen during pregnancy is present in bound, inactive form, so that it may not be available to the gingivate to the expected extent, and hence does not have a proportional effect on the gingivae. (2) There may be a reduced ability of the oral tissues to utilize the available estrogen because of the increased amounts and modifying effect of progesterone. (3) The large amounts of chorionic gonadotropin may affect the gingivae directly or indirectly. (4) The increased activity of the thyroid gland and the adrenal cortex may modify the effect of available estrogen on the gingivae. (5) Nutritional factors may alter the amount of endogenous estrogens available for the gingivae. These factors indicate the probable causes of the gingival alterations reported here.