We report a case of primary mucosa-associated lymphoid tissue (MALT) lymphoma of the prostate. A 67-year-old man presented with urinary obstruction and an elevated prostate-specific antigen (PSA) level. A physical examination revealed mild prostate enlargement and no lymphadenopathy. A needle biopsy and immu-nohistochemical studies of the prostate were performed, which revealed marginal zone B-cell MALT-type lymphoma. A bone marrow aspiration and biopsy did not show involvement by lymphoma. Magnetic resonance imaging (MRI) of the abdomen and the pelvis revealed no lymphadenopathy or ascites. There was no involvement of other sites by lymphoma. The patient was diagnosed and staged as extranodal marginal zone B-cell MALT-type lymphoma of the prostate, low grade and stage I. The patient received external beam radiation therapy to the prostate with a total dose of 3600cGy in 22 fractions, and became free of disease within the following 15 months.