Endometriosis is a pain syndrome representing a major cause of pelvic pain in women of reproductive age. The aim of this study was to test the hypothesis that persistent nociceptive input from endometriotic tissues leads to central sensitization manifested by somatic hyperalgesia and increased referred pain areas to experimental saline-induced muscle pain in patients with endometriosis, compared to healthy control subjects. Ten women with laparoscopically confirmed endometriosis and 10 healthy, age-matched women participated in the study. Hypertonic saline (0.5 mL, 5.8%) was injected intramuscularly, in random succession, into 1 site of menstrual pain referral (the multifidus muscle at the low back) and into 1 non-pain control site (first dorsal interosseous muscle [FDI] of the hand). The post-saline pain intensity and pain areas at the FDI were significantly greater in patients with endometriosis than in control subjects (P <.05) but were not different between the groups for the back. An absence of enhancement of post-saline pain responses at the back in the endometriosis group suggests that saline-induced pain at the back appears to activate segmental inhibitory systems in patients with endometriosis. Manifestation of central sensitization in women with endometriosis is demonstrated by increased muscle nociceptor input in the form of increased post-saline pain intensity, pain areas at the FDI, and hypersensitivity to pressure stimulation. These findings provide new insights into the complex pain mechanisms associated with endometriosis.