A 62-year-old woman presented with nausea, weakness, and vasovagal attack. She had a history of a carcinoma of the endometrium 19 years prior to admission, and adenocarcinomas of the cecum, ascending colon, transverse colon, rectum and sigmoid during the subsequent years. Due to the typical history, including family history, hereditary nonpolyposis colorectal cancer (HNPCC) was diagnosed 6 months prior to admission. Physical examination was normal with the exception of pallor of the skin and the palpebral conjunctivas. Laboratory values showed hypochromic, microcytic anemia (hemoglobin: 6.5 g/dl; MCV 70.1 fl; MCH 22.8 micrograms). Esophagogastroduodenoscopy revealed an exophytic mass protruding into the lumen of the inferior part of the duodenum. Carcinoma of the duodenum as a manifestation of HNPCC was diagnosed and partial duodenectomy performed. Macroscopic and microscopic examination revealed two adenocarcinomas--next to one another--7.0 cm and 2.5 cm in diameter. HNPCC is often overlooked and its relevance underestimated. Since diagnosis of this disease has major implications, all patients with colorectal cancer, and also young women with carcinoma of the endometrium, should be screened for HNPCC.