We describe an exceedingly rare case of severe gastritis that was temporally associated with primary Epstein-Barr virus (EBV) infection. The patient was a 59-year-old immunocompetent man who presented with intermittent fever of unknown origin and epigastric pain for 18 days. A computed tomographic scan of the abdomen showed diffuse thickening of the gastric wall and esophagogastroduodenoscopy revealed numerous ulcers in the stomach. Histologic examination of gastric biopsies showed a dense and diffuse atypical lymphoid infiltrate in the lamina propria with erosions and focal lymphoepithelial lesions. No lymphoid follicles or Helicobacter microorganisms were identified. Immunohistochemical studies demonstrated the lymphoid infiltrate to consist of mixed T and B cells. Immunoglobulin heavy chain gene arrangement analysis showed a polyclonal pattern. The plasma cells present in the biopsies exhibited no light chain restriction as determined by in situ hybridization. Concurrent clinical work-up revealed peripheral lymphocytosis with atypical lymphocytes and positive serum IgM antibody to EBV capsid antigen in the absence of IgG antibody. These findings indicated that the gastric abnormalities were related to primary EBV infection as the predominant manifestation of infectious mononucleosis. This was further confirmed by subsequent in situ hybridization showing numerous EBV-positive lymphocytes in the gastric mucosa. The patient's symptoms were spontaneously resolved with only supportive treatment. A follow-up endoscopy 2 months later showed completely normal gastric mucosa and he remained well with no gastrointestinal complaints for 2 and a half years. This case illustrates the importance of a high index of suspicion to avoid misdiagnosis of gastric lymphoma that requires more aggressive therapies.