A number of reports have measured NK cell activity in patients with endometriosis with varied results. Therefore we have examined the NK activity of PBL from 44 gynecological patients undergoing laparoscopy. This analysis has demonstrated a significant reduction in NK activity only in more severe stages of endometriosis (stages III and IV) relative to patients with milder disease and controls. These data indicate that decreased NK activity is unlikely to be a primary etiological factor in the development of endometriosis but may indicate that decreased NK activity is related to the development of the more frequent and/or larger lesions characteristic of severe endometriosis. These data could indicate potential for immunotherapy of patients with advanced endometriosis by the upregulation of NK activity in vivo.