An in vitro model developed to compare human endometrial and endometriosis stromal cells was used to examine basal and stimulated expression of interleukin (IL-6). Stromal cells isolated from normal endometrium (NE) exhibited the lowest level of IL-6 secretion (84 pg/106 cells-48 h), whereas those cells isolated from endometriosis implants (EI) secreted the highest concentration of this inflammatory cytokine (46,264 pg/106 cells · 48 h; P < 0.01). Eutopic endometrial stromal cells from women with endometriosis (EE) expressed an intermediate concentration of IL-6 (831 pg/106 cells · 48 h). Stimulation of the various cultures with IL-1β dramatically augmented stromal cell production of IL-6. The mean concentrations of stimulated IL-6 secretion were 16,257, 37,800, and 264,290 pg/106 cells · 48 h for NE, EE, and EI cells, respectively (P < 0.03). Exposure of the cell cultures to 10 nmol/L estradiol had little direct effect on IL-6 production. The results indicate that endometrial stromal cells isolated from tissues of women with and without endometriosis express IL-6 under basal and cytokine-stimulated conditions. Differential responsiveness among the three cell sources indicates that NE, EE, and EI cells have intrinsic quantitative differences in cytokine regulation.