SummaryThe authors describe a case of masculinisation in a 26 year old woman in whom an hypertrophic ovarium was found. The urinary excretion of 17 Keto- steroids was much increased. The surgical removal of that ovarium was followed by an important drop of the urinary 17 Ketosteroids.The histological examination of this organ proved that there were no signs of arrhenoblastoma nor of masculinovoblastoma. The hypertrophy of that ovarium was due to a highly increased number of luteinic cells in full activity.The conclusion of the authors is that one must consider a third group of masculinisation caused by an abnormal ovarium. They propose to call it«Luteinism»of the ovarium.