Use of gamma-linolenic acid and related compounds in the treatment of endometriosis

Method of prevention or treatment of endometriosis wherein effective amounts of one or both of i) gamma-linolenic acid and/or dihomo-gamma-linolenic acid and ii) 18:4 n-3 and/or one or more of the metabolites of 18:4 n-3 (20:4 n-3, 20:5 n-3, 22:5 n-3 or 22:6 n-3) are administered against endometriosis as such or in the form of an ester, salt, amide or other derivative convertible in the body thereto, alone or in an acceptable pharmaceutical carrier or diluent.