The cytologic findings in a fine-needle aspiration sample are described from a large retroperitoneal mass in a 56-yr-old male. The aspiration was performed under CT guidance using a 22-gauge needle, maintaining negative pressure. For cytologic study, on-site smears were prepared and stained by the Papanicolaou method. Additionally, cytospin, filter preparations, cell blocks from the aspirate, histology, and electron microscopy of the tumor tissue were performed. Also, immunodiagnostic staining for neuron-specific enolase (NSE), chromogranin, S-100 protein, vimentin, HMB45, cytokeratin, and Grimelius preparation was performed on cytologic and histologic material. The cytologic material was characterized by cords, suggestive acinar structures, and small follicle-like clusters of cells, while cell blocks from the aspirate showed oval or spindle-shaped nuclei with a somewhat fasciculated appearance. However, sections from the tumor tissue showed an organoid "zellballen" pattern. The nuclei were round, oval, spindly, and epithelioid, with moderate to scanty cytoplasm, smooth nuclear membranes, fine, evenly dispersed chromatin, slight hyperchromasia, and mild pleomorphism. No intranuclear vacuoles were seen. Positive immunohistochemical staining for NSE, chromogranin, and Grimelius preparation was noted in the tumor cells, while staining for vimentin, HMB45, and cytokeratin was negative. Electron microscopy of the tumor tissue revealed the presence of variable numbers of round, membrane-bound, electron-dense neurosecretory granules. The cytohistologic and ultrastructural findings are presented, as well as the results of immunodiagnostic staining which helped in the diagnosis of retroperitoneal paraganglioma, an infrequently reported tumor.