The association of mediastinal germ-cell tumours (MGCTs) with haematologic neoplasms is a rare though well known circumstance, and few cases are found in the literature. Most of these refer to non-seminomatous tumours in young males. The diagnosis of the haematological condition is usually either synchronic or metachronic with that of the germ-cell tumour. From those cases that have been published, we know that the prognosis is poor and basically determined by the haematologic neoplasia. The case report we present is that of a young male with an initial diagnosis of both conditions. It was possible to apply specific treatment, initially in the case of the leukaemia, and later in the case of the germ-cell tumour. The approach adopted is a multidisciplinary one.