A 52-year-old woman presented with low grade fever and fatigue. She had diffuse micronodules in both lung fields on chest X-ray. Chest CT showed diffuse multiple small nodules. Laboratory examination revealed high values for C-reactive protein, together with anemia and polyclonal hyper-immunoglobulinemia and an elevated interleukin-6 level. Although we suspected multicentric Castleman's disease, thoracoscopic lung biopsy revealed primary pulmonary MALT lymphoma by immunohistochemical analysis of tissue specimens. After COP and rituximab therapy, partial remission was obtained.