A Leydig cell tumor characterized by large tumor size, the presence of cytologic atypia, increased mitotic activity, necrosis, and vascular invasion. Approximately 10% of the testicular Leydig cell tumors show malignant characteristics and metastasize. Leydig cell tumors of the ovary follow a benign clinical course. The disease is MONDO_0000377 (malignant Leydig cell tumor). Also known as: Leydig cell cancer, Leydig cell tumor, malignant, cancer of Leydig cell, malignant Leydig cell neoplasm, malignant interstitial cell neoplasm, malignant neoplasm of Leydig cell.