A 62-year-old asymptomatic woman had a nodular shadow approximately 3 cm in diameter in the middle lung field on chest roentgenogram. The shadow gradually grew over 1.5 years. She was then admitted to our hospital for evaluation of the abnormal shadow. Transbronchial biopsy specimens showed a diffuse, small, lymphoid infiltrate with predominance of B-cells. These findings suggested neoplastic proliferation. Further examinations revealed the presence of Sjögren's syndrome. Four months later, she was readmitted for surgical resection. Chest roentgenogram on the second admission disclosed that the mass shadow had shrunk slightly. Histopathological examination of the resected specimen led to the diagnosis of malignant lymphoma, diffuse small cell type (IgG kappa type). Gene analysis revealed the presence of heavy chain gene rearrangements, which confirmed the diagnosis of B-cell lymphoma. In this case, no contraction or necrosis of the lesion was observed, and the change in the shadow size was probably due to partial or transient spontaneous regression.