Primary gastric diffuse large cell lymphoma is one of the most common extranodal lymphomas of the gastrointestinal system. Diagnosing gastrointestinal lymphomas can be difficult, since there is no pathognomonic sign in endoscopy to distinguish it from other malignancies. In some cases biopsy can be non-diagnostic. Therefore, multiple endoscopic examinations and biopsies can be necessary. With using confocal endomicroscopy, histology of the tissue can be seen in vivo and a range of diseases can be identified by using this technique. We are presenting a case, which is diagnosed as primary gastric diffuse large cell lymphoma during the evaluation of erythema nodosum etiology. We want to emphasize the role of confocal laser endomicroscopy for in vivo diagnosis of gastric lymphoma and directing the endoscopist for sampling the diseased mucosa. Confocal endomicroscopy decreases non-diagnostic rates in endoscopic biopsy and can be performed successfully in cases of gastric lymphoma. Pit patterns of gastric lymphoma, ring cell gastric carcinoma and gastric adenocarcinoma are similar. To best of our knowledge, this case is the fifth case of confocal laser endomicroscopy aided in diagnosing gastric lymphomas (Tab. 1, Fig. 2, Ref. 13).