Mast cells in the human uterus and adnexa have been studied using basic lead acetate fixation and a long toluidine-blue technique to maximise the numbers of cells stained. Counts were performed on measured areas of tissue and the numbers of mast cells related to clinical and pathologic variables. Considerable variation in numbers was found among individual cases at all the sites studied. In the endometrium and myometrium, a drop in the number of mast cells has been demonstrated with advancing age, particularly after menopause. In leiomyomas the highest counts were in the smaller and more cellular lesions. It is concluded that the numbers of mast cells are at least partly related to the degree of cellularity or atrophy of the surrounding tissues. No significant association was found with menorrhagia or with the presence of leiomyomas.