Three patients had autopsy-proved peritoneal lymphomatosis with ascites. Ascitic fluid analysis was characteristic in that the total protein level was greater than 2.5 g/dL, the lactate dehydrogenase level was greater than 225 mU/mL (the upper limit of normal for serum), and the glucose level was less than 50 mg/dL in all patients. Atypical cells were noted on ascitic fluid cytologic studies, and peritoneoscopic biopsy specimens were diagnostic of lymphoma in all three cases. Gut ulceration was present in all patients; a gastric ulcer, a duodenal ulcer, and a colonic ulcer were found to have invasion by lymphoma at autopsy. No patient lived long enough to receive chemotherapy. Perhaps if the diagnosis of lymphoma could have been made earlier, their lives could have been prolonged.