A primary cell culture of endometriotic cells was established to examine the hormonal regulation of endometriosis. The addition of estradiol had no effect on the DNA synthesis of cultured endometriotic cells. Progesterone at 10(-7) M caused a significant reduction in the amount of the DNA synthesis. An inhibitory effect of progesterone was completely eliminated by the concomitant addition of RU486, an antiprogesterone. The addition of epidermal growth factor (EGF) at 1 ng/ml significantly increased the DNA synthesis. Other growth factors, such as fibroblast growth factor and insulin-like growth factor also produced a modest but significant increase in DNA synthesis. These results seem to provide clues for clarifying the pathophysiology of endometriosis.