Abstract The numbers of mast cells were found to be fairly constant in the myometrium of the corpora uteri, and not to vary significantly with age or phase of the menstrual cycle. Reduced numbers of mast cells were present in gravid uteri. Considerable variation in numbers of mast cells occurred in the leiomyomatous tumors but the average counts were below those of the adjacent normal corresponding tissues. The average counts in leiomyosarcomas were about twice those in leiomyomas. There were no consistent findings to explain the high values encountered in some of the leiomyomatous tumors, although 2 such tumors had areas of extensive necrosis.