The effect of melatonin on endometrial cancer cell growth was investigated using two cell lines, SNG-II and Ishikawa, which are different in their estrogen receptor status. A physiological concentration of melatonin (10−9 M) showed no growth inhibitory effect on SNG-II cells, which are estrogen receptor-negative at all cell densities and incubation times. In contrast, melatonin significantly inhibited Ishikawa cells, which are estrogen receptor-positive at all cell densities tested after 96 hr incubation. The greatest inhibition of Ishikawa cell growth was observed at 10−9 M melatonin, compared with other supra (10−6, 10−8 M) or subphysiological concentrations (10−10, 10−12 M). This growth inhibitory effect of melatonin on Ishikawa cells was completely blocked by 10−10 to 10−8 M concentrations of 17-β estradiol administration. Pretreatment with luzindole, which is a selective melatonin receptor antagonist, prior to the addition of melatonin also blocked the inhibitory effect of melatonin on Ishikawa cells. This is the first study to demonstrate an anti-proliferative effect of physiological melatonin on endometrial cancer cells in vitro. The present study revealed that melatonin also inhibits the growth of estrogen receptor positive endometrial cancer cells and that this effect of the pineal indole may be mediated by both steroid and melatonin receptors.