Smears were made from bronchial washings of patients undergoing general endotracheal anesthesia. Cyclic cytomorphologic changes were observed in the ciliated columnar epithelia of younger females but not in epithelia of males and older women. During the proliferative phase of the menstrual cycle, the nuclei of the ciliated cells were found mainly in the basal position. Following ovulation they ascended to mid-position, and as the secretory phase progressed they migrated towards the apex. After approximately the 25th day, increasing numbers of cells with basal nuclei were again noticed. Intracytoplasmic mucopolysaccharide content increased with the ascent of the nucleus. When cells with basal nuclei predominated once more, goblet cells proliferated concurrently, probably indicating metaplasia from ciliated cells with high mucopolysaccharide content. These cyclic fluctuations may possibly protect females from inhaled irritants and carcinogens.