We examined a 63-year-old female, who was referred to our hospital for a thorough examination of abnormal findings on the X-ray which were first detected in May, 1992, and suggested a tumor. After hospital admission, a bracheobronchial lymph node biopsy was performed, and the results suggested a pseudo-lymphoma. However, because the possibility of a malignant tumor was not completely negated and the patient, as well as her family, hoped to receive surgical treatment, we performed the tumor resection. The tumor had a clear border. S6 was 3 x 3.5 x 4 cm, S8 was 2 x 2.5 x 3 cm, and evaluation according to the tumor node metastasis (TNM) classification was T1 N0 M0, i.e., Stage I. In addition to an ordinary histopathological examination, an immunohistochemical examination was conducted for CD19+, CD20+, CD22+, IgD-, CD5-, CD10-. Overall, the tumor was diagnosed as a MALT lymphoma. Investigated IgH, TCR beta, bcl-1 and bcl-2 genes did not show reconstitution. Because it is difficult to differentiate a malignant lymphoma, primarily developed in the lung, from a MALT lymphoma on a morphological examination of a tissue specimen, immunohistochemical and genetic examinations are helpful for making a diagnosis.