Multiple endocrine neoplasia type 1 (MEN1) and type 2 (MEN2) are rare autosomal-dominant disorders characterized by primary tumours in at least two different endocrine tissues. Both syndromes present as sporadic (a single case with two of the characteristic endocrine tumours) or familial form (an MEN case plus at least one first-degree relative showing one of the characteristic endocrine tumours). MEN1 is characterized by the occurrence of parathyroid, gastro-entero-pancreatic and anterior pituitary tumours, but it can include various combinations of more than 20 endocrine and non-endocrine tumours. Generally, tumours in MEN1 are benign, although gastrinomas and foregut carcinoids may exhibit a malignant course. MEN2 is characterized by medullary thyroid carcinoma (MTC), uni- or bi-lateral pheochromocytoma, and other tumours of different endocrine tissues. If not diagnosed precociously, MTC can be fatal. MEN1 develops after tissue inactivation of both MEN1 gene copies. Activating mutations of c-RET proto-oncogene causes MEN2.