Mucosa-associated lymphoid tissue (MALT) lymphoma commonly presents in the gastrointestinal (GI) tract. Supradiaphragmatic MALT lymphoma is less common and its natural history is not well defined. This study was conducted to understand the natural history, to determine the frequency of synchronous disease in the GI tract, and to understand the patterns of disease progression after treatment for supradiaphragmatic MALT lymphoma. We retrospectively reviewed the medical records of 39 patients who presented with supradiaphragmatic MALT lymphoma between 1991 and 1997. The median age of patients was 58 years (range, 25-90 years) with 16 male and 23 female patients. The most common primary site was salivary gland followed by ocular adnexa, lung, oral cavity, and others. Sixteen patients underwent esophagogastroduodenoscopy and biopsy (EGD + Bx) and 4 were found to have gastric involvement. Ann Arbor stages were the following: IEA, 17; IIEA, 5, IIEB, 1; and IVA, 16. The initial treatments were: involved field radiation therapy (n = 10), chemotherapy (n = 14), combination of radiation therapy and chemotherapy (n = 9), observation after biopsy (n = 4), antibiotics only (n = 1), and patient refusal of further intervention (n = 1). Seven patients received antibiotics as a part of the initial treatment. Every patient except for 1 was alive at a median follow-up of 39.5 months (range, 3-83 months). Thirty-six patients achieved complete response (CR) to the initial treatment. The actuarial 5-year progression-free survival rate was 83%. Progression of the disease occurred in 4 patients, with 2 in the stomach. Salvage attempts were made to 4 and were successful in 3. Of the 2 patients who relapsed in the stomach, 1 had negative EGD + Bx at the time of initial diagnosis. An EGD + Bx was not done in the second patient. Supradiaphragmatic MALT lymphoma appears to have a favorable prognosis. However, routine evaluation of the stomach is recommended for patients who present with supradiaphragmatic MALT lymphoma at the time of initial evaluation and at the time of relapse. Patients who failed initial therapy can be successfully salvaged with further treatment.