We report a case of a hCG-producing testicular seminoma revealed by a male infertility due to oligozoospermia. No palpable tumour was found at clinical examination and the diagnosis was established on hormonal data and the echography of the testis. The endocrine patterns associated high levels of beta-hCG and 17 beta-oestradiol contrasting with low levels of LH and FSH and normal levels of testosterone. Immunohistochemical studies confirmed the ectopic production of hCG. Histological findings and the evolution of hormonal parameters suggested that hyperoestradiolaemia was probably the consequence of a hCG-evoked Leydig cells hyperplasia involving a paracrine mechanism. In fact, after removal of the tumour, a dramatic decrease of plasma sex steroid levels was observed before recovery of normal testicular endocrine and exocrine functions.