Decidualized human endometrial stromal cells and human decidual CD56+ cells are reported to produce macrophage colony-stimulating factor (M-CSF), although its physiological roles remain unclear. In this study, the effects of M-CSF on cell viability and 8-Br-cAMP-stimulated decidualization of normal human endometrial stromal cells are examined by using an in vitro decidualization activity assay system. M-CSF alone affected neither the viable cell numbers nor PRL release of nonstimulated stromal cells, while high concentrations of M-CSF strongly suppressed the viable cell numbers and PRL secretion of stromal cells costimulated with 8-Br-cAMP and M-CSF. However, M-CSF did not suppress the viable cell numbers and PRL secretion of 8-Br-cAMP-induced decidualized cells. These results indicate that high concentrations of M-CSF can suppress the cell viability of stromal cells in the decidualization process, and inhibit decidualization of endometrial stromal cells. Thus, nonpregnant decidual M-CSF may autoregulate decidualization and the viable cell numbers of endometrial stromal cells in a paracrine manner.