The cytologic and histologic findings are reported in a case of papillary carcinoma arising within a thyroid gland pathologically enlarged by histiocytosis X. Fine needle aspiration (FNA) biopsies of a thyroid nodule in a patient with longstanding histiocytosis X produced a scanty amount of colloid, a moderately dense mixed inflammatory infiltrate and numerous small papillary fragments lined by cuboidal-to-columnar cells. Both the inflammatory cells and the epithelial cells showed nuclear grooves; the two populations of neoplastic cells were distinguished, and the correct diagnosis of the nodule was made, by recognizing the greater amount of granular cytoplasm of the mononucleated Langerhans' cells. The rare involve-of the thyroid by histiocytosis X is reviewed; this case, which appears to be the first reported instance of the co-occurrence of histiocytosis X and papillary carcinoma of the thyroid, indicates that patients with histiocytosis X should be observed for the development of thyroid carcinoma and that FNA biopsy can make the distinction between the two conditions.