leydig cell tumours are rare stromal tumours of the testis accounting for 13% of testicular neoplasms . they exhibit a peak incidence in the preadolescent as well as in the older ( > 50 years ) age groups [ 1 , 2 , 3 , 4 ] . the diagnosis of a malignant leydig cell tumour is not always easy since no definite histological criteria for malignancy exist . a 35-year - old man presented to our hospital with a 3-month history of a painless left testicular mass . blood concentrations of chorionic gonadotrophin , fetoprotein and human placental lactogen were within the reference range . a left radical orchidectomy was performed and the specimen submitted for histopathological examination . on microscopic examination , a malignant leydig cell tumour was found . the cells displayed acidophilic cytoplasm , intranuclear inclusions and increased mitotic activity ( > 3/10 hpf ) . many cells with large atypical pleomorphic nuclei could also be observed ( fig . 1 , fig . 2 ) . there was no angiolymphatic invasion , foci of necrosis or extension beyond the capsule of the testis . the immunohistochemical study showed that the tumour cells were positive for vimentin , melan a and inhibin , and negative for ckae1 , ckae3 , s100p , p63 , cea , afp and actin . 3 ) . retroperitoneal lymph node dissection was suggested , but the patient declined further surgery . leydig cell tumours are rare and only about 3% of them are bilateral . about 1520% of the patients already present with metastatic disease , particularly in the lymph nodes , lung and liver . these tumours may be hormonally active , and gynaecomastia is seen in 30% of the cases [ 2 , 3 , 6 ] . leydig cell tumours in an undescended testis may exhibit only manifestation of endocrinological disorders such as gynaecomastia , impotence and loss of libido . however , among the 480 cases of leydig cell tumours reported in the literature , only 20 cases were associated with cryptorchidism , and there is no evidence that undescended testes are more prone to develop leydig cell tumours [ 2 , 5 , 7 ] . features associated with malignancy include a large tumour size ( 5.7 cm ) , nuclear atypia , a mitotic count of > 3/10 hpf , foci of necrosis , angiolymphatic invasion , infiltrative margins , dna aneuploidy and an increased expression of ki67/mib-1 and p53 [ 2 , 3 , 8 , 9 , 10 ] . our patient was young and did not fit into either of the known age incidence peaks . microscopic features such as the severe nuclear atypia and increased mitotic activity ( > 3/10 hpf ) favoured the diagnosis of malignancy . however , there was no angiolymphatic invasion or extension beyond the capsule of the testis . also , the small size of the tumour was remarkable .
leydig cell tumour is a benign testicular non - germ cell tumour , and malignant transformation is rare . 
 we report a case of a 35-year - old man who came to our hospital with a painless left testicular mass measuring 1.2  1 cm . 
 histological evaluation of the tumour showed features of a malignant leydig cell tumour but no infiltration beyond the capsule or metastasis . 
 the small size of the tumour was remarkable .