Pseudomelanosis duodeni is a rare benign condition. It manifests endoscopically 
as discrete, flat, small brown-black spots in the duodenal mucosa. It produces 
no symptoms and may be reversible. The cause and natural history of the 
pigmentation have not been clarified, although it is associated with a variety 
of systemic illnesses and medications. With electron microscopy and 
electron-probe energy dispersive X-ray analysis, the pigment corresponds 
principally to an accumulation of ferrous sulfide (FeS) in macrophages within 
the lamina propria. We report the case of a 56-year-old female patient with a 
past history of diabetes mellitus and hypertension. She was admitted because of 
nausea, vomiting, and diarrhea and underwent esophagogastroduodenoscopy because 
of stool occult blood test results of 3+. Endoscopy revealed diffusely scattered 
black spots in the bulb and second portion of the duodenum. Histological 
examination showed numerous pigment-laden macrophages in the lamina propria of 
mucosal villi. The diagnosis requires further confirmation by electron 
microscopy and electron-probe energy dispersive X-ray microanalysis. No special 
therapy is indicated for this rare lesion.