Multiple intra-abdominal organ infiltration or disseminated peritoneal lymphoma receives much less attention than peritoneal carcinomatosis in clinical practice. This may be due to its relatively infrequent occurrence. In this report, an 89-year-old woman was diagnosed with disseminated peritoneal lymphoma with gastric and rectal involvement and marked ascites. Flow cytometry of the surface markers for ascites showed positive results for CD19, CD20 and CD45. Biopsy of the stomach and rectum were all reported to show diffuse large B-cell lymphoma. The cytology of ascites is a simple and effective method for making a diagnosis from adequate samples with time limitations. The management of this disease depends on the individual case. It must be kept in mind that differential diagnosis from other pathologic entities with similar imaging features or high ascitic fluid adenosine deaminase levels is difficult because of considerable overlap of clinical features. To prolong the survival of patients with peritoneal lymphomatosis, diagnosis should be made as early as possible.