The histologic features and lectin binding patterns of endometrial tissue obtained in 16 cases of inadvertent biopsy during the cycle of conception were studied. Six biotinylated lectins that bind to specific glycoconjugates of secretory or gestational endometrium in paraffin-embedded tissue were used. These lectins were wheat germ agglutinin (WGA), Ricinus communis agglutinin-I (RCA-I), Concanavalin A agglutinin (Con-A), peanut agglutinin (PNA), Dolichos biflorus agglutinin (DBA), and soybean agglutinin (SBA). On review, the histologic sections showed day 20 to 26 patterns. In retrospective analysis, 15 of the 16 cases had increased stromal edema and vascular congestion compared with endometria of equivalent postovulatory dates not associated with conception. In 2 cases, spiral arterioles showed increased thickness. There was no evidence of renewal or prominence of glandular secretions. Most lectin binding was similar to that of the secretory phase of the menstrual cycle (WGA, RCA-I, gland lumenal secretions and stromal granulocytes; Con-A, gland cell cytoplasm and predecidua). There was focal, irregular staining of gland secretions with PNA, DBA, and SBA, lectins that generally react only with gland secretions in later gestation. These results indicate that vascular changes including edema and congestion appear to be the earliest persistent morphologic manifestations of the pregnant endometrium. Secretory gland changes, however, are subtle and are not identifiable by routine histology.