A 61-year-old patient had the clinical findings of Pancoast syndrome with shoulder pain, weakness and atrophy of band muscles, and an opacified area in the apical lung field. Unlike most cases with Pancoast syndrome in which the tumors are primary carcinomas of the lung, the present case was shown to have a plasmacytoma involving the apical parietal pleura and the adjacent chest wall. The need for histologic diagnosis before treating patients with Pancoast syndrome is emphasized.