In endocervical smears obtained from 40 women with Trichomonas vaginalis infection, 25 women with infections other than T. vaginalis and 35 cytologically normal women, inflammatory cells consistent with mast cells (tissue basophils) were examined. Mast cells were present predominantly in the smears of patients infected with T. vaginalis but were infrequent in those of patients with other infections and in those of the normal, control group. The numbers of mast cells in endocervical smears and the white blood cell counts in vaginal discharges showed a positive correlation with the degree of T. vaginalis infection. Our data seem to indicate that the presence of mast cells in the uterine cervix may be an immunologic response, possibly of an allergic nature, to T. vaginalis.