We report a case of a 28-yr-old man in whom mastocytosis was diagnosed by fine-needle aspiration (FNA) of enlarged periaortic lymph nodes. FNA cytology revealed numerous mast cells characterized by cells having round to oval eccentrically placed nuclei and associated coarsely granular cytoplasm, most conspicuous on Diff-Quik- and Giemsa-stained smears. Lymphocytes, histiocytes, and occasional plasma cells were present in the background. We believe this paper to be the first reported case of mastocytosis diagnosed by FNA. The case demonstrates the utility of FNA in evaluating lymphadenopathy occurring in mastocytosis.