A case of inflammatory pseudotumor of the lung occurring in a six-year-old boy is reported with clinicopathologic findings, including its ultrastructure. The patient had had frequent upper respiratory tract infections, and one and half year before the discovery of the lung mass, he suffered from pneumonia of the right lung, which was serologically proven to be a mycoplasma pneumoniae infection. Exploratory thoracotomy revealed a large mediastinal mass that was removed together with the right middle and lower lobes of the lung. The mass arose from the lung with an endobronchial element. Microscopically, the mass was composed of a variety of inflammatory and mesenchymal cells, including plasma cells, histiocytes, lymphocytes, and fibroblast-like spindle cells. Ultrastructurally, the spindle-shaped mesenchymal cells were either fibroblasts or myofibroblasts. At the time of diagnosis of the inflammatory pseudotumor of the lung, the serum titer of antimycoplasma antibody rose again, and the lung parenchyma adjacent to the mass showed interstitial pneumonia with features of bronchiolitis obliterans. The present case suggests that the inflammatory pseudotumor of the lung could be a postinflammatory lesion associated with mycoplasma pneumoniae infection.