A fine needle aspiration biopsy specimen of a retroperitoneal mass in a 26-year-old man who had had an orchiectomy for seminoma was submitted for cytologic evaluation. Cytologic features of the specimen included uniform neoplastic cells found singly or in groups of several cells intermingled with lymphocytes in a characteristic foamy, lacelike background. These cells varied from 10 to 20 m in diameter. The nuclei were round to ovoid with fine or reticular chromatin and one or more prominent nucleoli. The poorly defined cytoplasm stained pale-blue or blue with cytoplasmic vacuoles. The cytologic appearance was consistent with seminoma. Documented reports of the cytological appearance of seminoma are rare. The diagnosis of primary gonadal seminoma by fine needle aspiration biopsy is probably not indicated since the treatment of a primary gonadal tumor, regardless of its histogenesis, requires surgical resection. However, fine needle aspiration biopsy is extremely valuable in the diagnosis of extragonadal as well as metastic and recurrent seminoma.