Primary malignant lymphoma of the lung is a very rare disease, which consists of 0.34% of entire malignant lymphoma. The majority are low-grade B-cell tumors, and because of their morphological peculiarities and overall excellent prognosis, many cases, like many other extranodal lymphomas, have been mislabelled as "pseudolymphomas" in the past. For these reasons their true incidence is difficult to estimate. An incidentally discovered mass in the right middle lobe of a 36-year-old woman was operated on November 9, 2001 at Hanyang University Hospital. A right upper lobectomy was done and the pathologic diagnosis of extranodal marginal zone B-cell lymphoma of MALT type was made.