Several cases of spontaneous endometriosis in middle-aged to old rhesus macaques (<i>Macaca mulatta</i>) from the breeding colony of the German Primate Center were thoroughly characterized with regards to anatomical distribution and macroscopic appearance, histological differentiation and immunohistochemical profile including somatic markers, hormonal receptors, and proliferation indices. The gene discussed is NR4A1; the disease is endometriosis.