Experience in the endoscopic and bioptic diagnosis of malignant non-Hodgkin's lymphomas of the stomach in 66 patients is reported all of which were B cell lymphomas originating in the mucosa associated lymphoid tissue (MALT) type. Two types of tumor could be differentiated by their appearance on endoscopy (two patients had both types). An exophytic type (n = 24) was easily recognized as malignancy on endoscopy and the diagnosis confirmed by endoscopic biopsy (mean of 1.2 endoscopic-bioptic examinations). It could be classified according to the Palmer classification; 50% were low-grade malignancies and 50% high-grade malignancies. Resection (n = 17) showed wall penetration beyond the muscularis propria in 70%. An infiltrative type (n = 44) was difficult to diagnose by means of endoscopy and biopsy (mean of 2.9 endoscopic-bioptic examinations); in one case a histological diagnosis could only be established on surgery. A classification system is suggested for this type (I--elevated, II--flat, III--cavitated). Most of these lymphomas were low-grade malignancies (77%), and limited to the mucosa and submucosa (79% of 34 resected cases). It is concluded that attention should be paid especially to the infiltrative type of gastric lymphoma which is difficult to diagnose and to differentiate from other gastric conditions such as ulcers and erosions, but which is most often found at an early stage and has a better prognosis. Complete tumor resection (R0) was achieved in 49 of 51 patients undergoing surgery (96%).(ABSTRACT TRUNCATED AT 250 WORDS)