Isolated primary tumors of the lung are rarely seen in childhood and are benign in one third of the patients. Plasma cell granulomas represent the most common benign tumor of the lung in this age group. Histologically, the tumor is characterized by the various components of a chronic inflammation or granulation tissue. Frozen sections can pose difficulties in the differentiation between a malignant and benign process. In view of the case history of our patient we set out to demonstrate that a knowledge of the clinical picture and a correct interpretation of associated laboratory findings facilitate diagnosis. To our knowledge we describe for the first time the ultrastructural findings of so-called "microtubule-reticular structures" giving further evidence of the inflammatory origin of this lesion.