The purpose of this study was to observe the safety of the prolonged use of the stainless steel ring. The endometria of 8 women who had had stainless steel ring insertion for 20 years with normal menstruation were examined under the electron microscope. The results were: the pressed area of endometrium showed various degrees of atrophy or degeneration; the adjacent area showed slight degeneration or mild proliferation of the surface epithelium, few cells with hypertrophy or metaplasia; no apparent malignant changes had been found. The remote area did not show any apparent pathological changes. It seems their hormonal reactions were relatively normal, and even sensitive structures such as N.C.S. and giant mitochondria still appeared. Thus, there seemed to be no severe endometrial destruction or malignant changes. In correlation with clinical analysis, it is suggested that the prolonged use of the stainless steel ring for as long as 20 years seems to be safe.