Endometriosis is generally regarded and treated as an endocrine and/or a surgical disease. Endocrine and surgical approaches to treatment are effective only in treating the symptoms of the disease and not its pathophysiology. Consequently, the success of such therapy is at best short-lived, and probably non-existent in mild disease. This thesis therefore argues for a new therapeutic approach towards endometriosis. In view of increasing evidence in favour of an immunological aetiology for this condition, it is suggested that such an approach may involve immunological rather than endocrine manipulation. It is also proposed that so-called unexplained infertility may in many cases represent a precursor stage of endometriosis, since immunological abnormalities in both of these conditions are so similar. Largescale, prospective, multicentre clinical trials of immunotherapy in unexplained infertility and endometriosis appear to be indicated at this time if endometriosis is to lose its importance as one of the major contributors to female infertility .