Immunohistochemical distribution of oestrogen receptors (ER), progesterone receptors (PR), and the cell proliferation-associated antigen Ki-67 was investigated in leiomyomas and the myometrium during the menstrual cycle and pregnancy. In the myometrium, ER expression was observed in the proliferative phase, but was suppressed in the secretory phase and during pregnancy. In leiomyomas, ER expression was observed throughout the menstrual cycle, but was suppressed during pregnancy. However, PR was expressed both in the myometrium and leiomyomas throughout the menstrual cycle and pregnancy. In both the myometrium and leiomyomas, a higher number of Ki-67-positive cells was observed during pregnancy than in the secretory phase, and Ki-67 was negative during menopause. The Ki-67-positive cell count in leiomyomas was significantly higher than that in the myometrium throughout the menstrual cycle and pregnancy. Thus both myometrium and leiomyomas have high growth activity under the hormonal milieu of high progesterone levels. The growth potential of leiomyomas is apparently higher than that of myometrium throughout the menstrual cycle and during pregnancy.