We report nine cases of uterine leiomyomas with an average of more than 10 mast cells per high-power field. The clinical presentation and morphological features of the tumors did not differ from those of conventional leiomyomas. Two cases also showed eosinophilia. The adjacent myometrium did not show an increased number of mast cells. Mast cells are frequently numerous in leiomyomas, but the significance of this finding is unknown.