The authors performed 592 histological examinations of the endometrium in contact with 5 different types of IUD. 4 parameters were considered, the purely mechanical action of the device causing lesions on the endometrial surface, hormonal modifications, vascular modifications, usually related to menorrhagia and/or metrorrhagia, and inflammatory modifications, which can be related to IUD presence, or caused by external infection. The analysis of the histological parameters of IUD action of the endometrium showed their very complicated interrelation; the endometrium does nto simply react to a foreign body, the IUD, but it is a hormonal-dependent tissue, thus representing a link in a large and complex hormonal- and neurosympathatic system, which is, as yet, not perfectly known.