Primary pulmonary lymphoma (PPL) is an uncommon tumor, which constitutes 3-4% of all extranodal lymphomas and 0.3-0.5 % of all primary pulmonary malignant tumors. Low-grade B-cell lymphomas of bronchus-associated lymphoid tissue (BALT) accounted for the majority of PPL. This BALT lymphomas are frequently asymptomatic and have an excellent prognosis and an indolent clinical course by contrast with T-cell type. Therefore, determination of the B- or T-immunophenotype of the tumor cells is known to be very important from a clinical aspect. Recent advances in immunohistochemical techniques, cytogenetics, and molecular biology have allowed better definition of type, maturation, and clonality of lymphoma cells and have made it possible to better understand the PPL. We experienced an asymptomatic 43-year-old man who was evaluated for infiltrates on both sides discovered incidentallly after a routine chest roentgenogram. He was eventually diagnosed as low-grade B-cell lymphoma of BALT by immunohistochemical staining from specimens obtained by open lung biopsy. He was treated with combination chemotherapy. At follow up 12 mons following initial diagnosis, he remains in stable. We report this case, who showed a relatively favorable prognosis and indolent clinical course compatible with low-grade B-cell lymphoma.