Increasing evidence suggests that macrophage colony-stimulating factor (M-CSF) is produced in the uterine endometrium and that it plays an important role in the reproductive process. In the present study, using an in vitro decidualization model and human endometrium, we investigated M-CSF messenger RNA (mRNA) expression in human endometrial stromal cells (ESC) by Northern blotting and in situ hybridization. The secreted M-CSF in the culture medium of ESC was measured by enzyme-linked immunosorbent assay. ESC were cultured in the presence of progesterone (P) or estrogen. After a 9-day culture with P, when in vitro decidualization was confirmed by the production of PRL, M-CSF mRNA and protein levels were 3.1 +/- 0.5- and 3.2 +/- 0.8-fold (mean +/- SEM) higher, respectively, than those in cultures without P (P < 0.01). The P-induced increase was dose dependent. On the other hand, estrogen did not increase M-CSF mRNA expression. M-CSF mRNA expression in the first trimester deciduae that expressed PRL mRNA was higher than that in the endometria. By in situ hybridization, ESC as well as epithelial cells were shown to express M-CSF both in vitro and in vivo. These findings indicate that human ESC (decidua cells) express M-CSF mRNA and suggest that they secrete M-CSF in a P-dependent manner during the process of decidualization.