Diseases of the male mammary gland are clinically manifested as unilateral or bilateral, well-defined or diffuse consolidations in the breast. In some cases, the affected tissues are extremely tender on pressure. Gynaecomastia is most frequently seen (n = 22), whereas inflammatory or tumourous changes of the glandular parenchyma are rare (n = 2). The most important causes of gynaecomastia are shifts in hormonal balance where the controlling impulses acting on the glandular parenchyma are enhanced. Gynaecomastia is frequently found in patients who have cirrhosis of the liver or are taking certain drugs. 29 cases are described from the author's own patient material. The radiological image, the clinical findings, the possible aetiological aspects, as well as histological findings, if any, are described and discussed in relation to literature. Closer examination of this poly-aetiological disease pattern--namely, swelling of the male mammary gland--leads to a number of interesting differential diagnoses which sometimes require internistic and endocrinological clarification before the correct diagnosis can be found.