THE. HE RARER lesions responsible for the radiographic appearance of solitary pulmonary nodule include plasma cell and mast cell "tumors." On routine histologic study, plasma cells may be indistinguishable from mast cells, suggesting the need for reevaluation of instances reported as "solitary plasmacytoma of lung." In addition, the discovery of the mastocytosis syndrome has spurred investigation of the mast cell and its diseases. It is the purpose of this paper to report the second case of solitary pulmonary mast cell granuloma and to discuss its place in the spectrum of mast cell disease as well as its relationship to plasmacytoma. Report of a Case A 68-year-old white woman entered University Hospital on Aug 4, 1964, for evaluation of an asymptomatic lung lesion. A routine chest x-ray film taken on July 2, 1964, had demonstrated a 2-cm nodule located just above the right side of the hilum (Fig 1A). A