Endometriosis has been shown to be associated with increased number and activity of peritoneal macrophages. The peritoneal macrophage-conditioned media from 33 women with or without endometriosis were studied for their effects on an endometrial carcinoma cell line, ECC-1. The media from six of six stage III/IV cases demonstrated a mitogenic effect, which was blocked by an antibody to epidermal growth factor receptor. However, the conditioned media from seven of nine stage I/II cases and 14 of 18 normal women did not show a mitogenic effect. The difference between stage III/IV and the other two groups was significant (p less than 0.01). The incorporation of tritium-thymidine was three times higher with the media from stage III/IV cases, as compared with that of controls. When purified cytokines were tested in the tritium-thymidine uptake assay, only epidermal growth factor-transforming growth factor-alpha was mitogenic on ECC-1, whereas tumor necrosis factor, interleukin-1, and platelet-derived growth factor had no effect. Thus peritoneal macrophages in patients with endometriosis may play an important role in the progression of endometriosis, and the noted effects could be mediated by epidermal growth factor or a related growth factor.