Inflammatory myofibroblastic tumor (plasma cell granuloma). Clinicopathologic study of 20 cases with immunohistochemical and ultrastructural observations. Twenty cases of inflammatory myofibroblastic tumor (IMT) were studied; 19 involved the lung and 1 the esophagus only. The patients' ages ranged from 3 to 72 years. There were 9 males and 11 females. Involvement of a bronchus was seen in one case and of mediastinal structures in four. Chest pain and dyspnea were common symptoms; eight patients were asymptomatic. Seven patients underwent lobectomy, 12 local excision, and 1 biopsy alone. The lesions were nonencapsulated and ranged from 1.2 to 15 cm. Various proportions of plasma cells, histiocytes, and spindle cells were observed; the latter corresponded ultrastructurally to fibroblasts and myofibroblasts, were immunoreactive for vimentin and actin and focally for desmin, and were negative for epithelial markers. Plasma cells were polyclonal for light chains. One patient had two recurrences, and in one case a large pleural IMT was found eight years after the excision of a similar lesion in the lung. All patients with follow-up (ten) were well as long as ten years after the diagnosis (average, 3.7 years). 