Testicular adrenal rest tumours are frequently associated with congenital adrenal hyperplasia (CAH). These ACTH-dependent tumours cannot be easily distinguished histologically from Leydig-cell tumours. We report the case of a 30-year-old man who was explored for infertility, azoospermia and unilateral testicular tumour. High levels of 17-OH progesterone and ACTH, low cortisol and undetectable gonadotropins levels, associated to bilateral adrenal hyperplasia, led to the diagnosis of CAH by 21-OH deficiency with a composite heterozygoty. The testicular tumour was first considered as adrenal rest. However, histological analysis of this unilateral painful tumour showed a steroid-hormone-secreting cell proliferation with atypical and frequent mitosis. To discriminate between a benign adrenal rest tumour and a possible malignant leydigioma, tumoral expression of specific gene products was analyzed by RT-PCR. No 11-beta-hydroxylase nor ACTH receptor mRNAs could be found in the tumour, which did not behave like usual adrenal rest cells. For this unilateral testicular tumour, the lack of adrenal-specific markers associated with a high rate of mitosis and pleiomorphism supported a leydigian origin with malignant potential. However, lack of tumoral LH-R mRNA expression and a tumour-free 3-year follow-up led us to retain the diagnosis of adrenal rest tumour with loss of adrenal gene expression and progressive autonomous behaviour.