The contrasting findings obtained in the studies that have attempted to correlate the stage of endometriosis with severity of pain symptoms suggest that some specific characteristics of the lesions are more implicated in the genesis of pain than disease extension per se. Thus, fresh, metabolically active, intraperitoneal implants may cause functional pain symptoms such as dysmenorrhea, whereas infiltrating, nodular andfibrotic lesions are responsible for organic-type pain such as deep dyspareunia. Women with symptomatic endometriosis seem to have reduced peripheral β-endorphin production in comparison with pain patients without the disease, although neuroendocrine modulation of pelvic nociceptive stimuli is far from clear. There is little evidence to support the notion that specific psychiatric features render some women more vulnerable to developing endometriosis, as results from investigations performed on women with asymptomatic lesions are very similar to normative data. Moreover, it appears that the psychological profile of symptomatic patients with the disease is no different from those with pain and a normal pelvis or other gynecological conditions. Consequently, the local biochemical and physical effects of lesions seem to be the most important factors in determining frequency and severity of symptoms.