A sex cord-stromal tumor occurring in the testis and rarely in the ovary. It is predominantly or completely composed of Leydig cells which may contain crystals of Reinke. In males it usually presents as a painless testicular enlargement and it may be associated with gynecomastia and decreased libido. The majority of the cases have a benign clinical course. Approximately 10% of the cases have a malignant clinical course and metastasize. In females it may be associated with androgenic manifestations and it follows a benign clinical course. The disease is Monarch Disease Ontology id MONDO_0006266 (Leydig cell tumor). Also known as: Leydig cell neoplasm, interstitial cell neoplasm, interstitial cell tumor, interstitial cell tumour.