Tissue culture techniques commonly applied to the study of human vascular smooth muscle were used to evaluate in vitro survival and proliferation of normal and neoplastic human myometrial cells. Despite their growth advantage in vivo, leiomyoma cells displayed a growth disadvantage in vitro compared with normal myometrium from the same patient. Hormonal supplementation with alpha-estradiol, progesterone, and insulin-stimulated myometrial proliferation, whereas beta-estradiol appeared ineffective at the doses tested. Hormonal supplementation also stimulated leiomyoma proliferation in vitro, but there appeared to be heterogeneity in hormonal responsiveness. Heterogeneity in the host hormonal milieu and in the ability of uterine leiomyomas to respond to various hormones may be important factors contributing to the wide variation in growth potential observed in leiomyomas.