A 14 year old, sexually intact, 3 kg in body weight, mixed breed queen was presented to Clinic of Department of Obstetrics and Gynecology, Faculty of Veterinary Medicine, Istanbul University with pink-red vaginal discharge, abdominal extension, anorexia, vomiting, polyuria and polydipsia. Hemogram and biochemical analyses revealed increased alanine aminotransferase (ALT), aspartate aminotransferase (AST), urea, creatinine, gamma-glutamyl transferase (GGT) and cholesterol levels as well as polycytemia. Ultrasonographic examination revealed pyometra and an intrauterine mass was suspected. Resected uterus and ovaries were examined at the Department of Pathology, Faculty of Veterinary Medicine, Istanbul University. Histopathological examination confirmed a malignant mesenchymal tumor (sarcoma) and cystic endometrial hyperplasia pyometra complex (CEH-P). It was concluded that malignant mesenchymal pathologies of uterus which are rarely seen in cats, can occur together with pyometra and they can be treated successfully with surgery.