The invention relates to a new prescription of NAC in the treatment of endometriosis and of indications associated with endometriosis, in a human or mammalian animal patient. In addition an effective dose regimen of NAC in the treatment of endometriosis is proposed. In one embodiment of the present invention the prescribed treatment regimen may be used e.g. in order to control the frequency and the intensity of pain symptoms (dysmenorrhea, dyspareunia and acyclic chronic pelvic pain), to reduce the size of endometriotic lesions eventually up to their disappearance, to reduce recurrences after surgery and/or to improve fertility. Side effects of this treatment are virtually absent and, in particular, this treatment does not hinder pregnancy.