A 37-year-old man was hospitalised because of anaemia and fatigue due to an infiltrating adenocarcinoma of the Treitz angle (duodenum), together with gastric, duodenal and colorectal polyps. After the operation, removal of colorectal lesions revealed the presence of ganglioneuromatosis of the large bowel. Further investigations showed lack of MLH1 protein expression and microsatellite instability in the duodenal neoplasm, while the gene was normally expressed in the polyps. MLH1 sequence and Multiple Ligation-dependent Probes Amplification analysis (from constitutional DNA) were normal. Analysis of the PTEN gene revealed the presence of a constitutional mutation (c.510 T>A; p.Ser170Arg) which had been associated with the Cowden phenotype. Further detailed clinical investigations revealed macrocephaly (63 cm), melanotic spots of the penis, small angiomas, millimetric trichilemmomas in the nose and multiple lipomas, which led to the diagnosis of Cowden/Bannayan disease. The unusual appearance of a duodenal carcinoma as the first symptom rendered the identification of the syndrome extremely difficult.