Background Malignant tumours of the duodenum are rare and often difficult to diagnose. Due to the small clinical experience with duodenal malignancies their prognosis is unknown and resection is the treatment of choice. Case report Adding to a small series of incidental tumours, we report the case of a 65-year-old patient with primary extranodal (MALT-) lymphoma of the duodenum infiltrating the pancreatic head. The patient was admitted because of anaemia and epigastric discomfort with a history of Helicobacter- pylori associated gastric ulceration. Physical examination and bloodchemical values were otherwise normal. Endoscopy revealed duodenal ulceration but the biopsies taken from the ulceration did not give any evidence of malignancy or residual Helicobacter pylori infection. But MRT showed a circular intramural tumour of the duodenum. On laparotomy a large duodenal tumour adherent to the pancreatic head was found and a Whipple procedure was performed. Conclusion Apart from describing the case of a rare lymphoproliferative disorder of the duodenum, this report illustrates the diagnostic difficulties with uncommon neoplasm's of the duodenopancreatic region and the value of MRT prior to resection of a duodenopancreatic mass.