The relationship between argyrophil nucleolar organizer region (Ag-NOR) protein quantity and prognosis was studied in 33 cases of stage I endometrial adenocarcinoma. Ag-NOR protein quantity was measured by image analysis in silver-stained sections from paraffin-embedded samples of curettings. Patients had a minimum 10-year follow-up. Only 2 out of 25 patients exhibiting a mean Ag-NOR protein area of less than 3 microns2 died of cancer, whereas 5 of the 8 patients with a mean Ag-NOR protein area of more than 3 microns2 died of the disease. The present results demonstrate that the Ag-NOR protein value is closely related to patient survival in stage I endometrial carcinoma and that it is a reliable prognostic indicator in this type of carcinoma.