Gynaecomastia, or enlargement of the male breast may result from various endocrine dysfunctions and often reflects ectopic production of substances such as hCG and estradiol. We report on the case of a 30 year old man who presented with gynaecomastia and elevated plasma levels of hCG, estradiol and testosterone. As a result of several diagnostic procedures such as selective venous sampling and magnetic resonance tomography (MRT), a hCG producing tumor of the upper lobe of the left lung was found. This hormonal overproduction induced an enhanced secretion of estradiol and testosterone in the testicular tissue. Histology revealed a giant cell carcinoma with positive immunostaining for hCG. This case report further underlines the necessity of an intensive search for ectopic beta-hCG production due to malignant tumours, in particular in the adult.