Endometrial hyperplasia is thought to be caused by the prolonged, unopposed oestrogenic stimulation of the endometrium. The regression of hyperplastic back to normal endometrium is the main purpose of any conservative treatment in order to prevent development of adenocarcinoma. The aim of this study was to evaluate the regression of hyperplastic to normal endometrium in patients with various forms of endometrial hyperplasia after treatment with the gonadotrophin-releasing hormone analogue (GnRHa) triptorelin for 6 months. Fifty-six patients with endometrial hyperplasia were enrolled in this trial; 39 patients (group I) presented simple hyperplasia, 14 (group II) complex hyperplasia and three (group III) atypical complex hyperplasia. All patients were treated with triptorelin for 6 months. Bleeding control during treatment was excellent. A post-treatment curettage for estimation of endometrial histology was performed on 54 out of 56 patients 100.1 +/- 44.7 days after the last triptorelin dose, following the restoration of pituitary function. Regression of hyperplastic to normal endometrium was observed in 32 (86.5%) out of 37 patients in group I and in 12 (85.7%) out of 14 in group II. Persistence of simple hyperplasia was found in five (14.5%) out of 37 patients in group I. Persistence of complex hyperplasia was found in 1 (7.1%) out of 14 patients and progression to atypical complex hyperplasia in another one (7.1%) woman in group II. In some of these cases, the presence of risk factors such as obesity, diabetes mellitus and ovulatory disturbances may contribute to the disease persistence despite therapy. On the other hand, in group III, none of the three patients had normal post-treatment endometrial histology. It seems, therefore, that in cases of endometrial hyperplasia without atypia, the administration of the GnRHa triptorelin is associated with high regression rates to normal endometrium. Conversely, the presence of atypia seems to be a poor prognostic factor. Treatment tolerance and bleeding control during therapy is excellent.