A 17-year-old male with effusion in the pleural cavity and a tumor on the left 7th rib was diagnosed by exfoliative cytological and histological examination of pleural fluid and biopsy specimen from the chest wall tumor. In the pleural effusion, small round cells with nuclear atypia were demonstrated as a mixture of monolayered clusters and single cells, with scanty and indistinct cytoplasm. The nucleus was round, supported by thin and smooth nuclear membrane, and hyperchromatic, showing finely granular chromatin pattern with one or two small but distinct nucleoli. PAS stain demonstrated intracellular glycogen. Histologically, biopsy specimen from the chest wall tumor yielded a diagnosis of round cell tumor, compatible with Ewing's sarcoma. Differential diagnosis of small round cell tumors was discussed from a morphological point of view.