The authors report the histories of two patients with undifferentiated carcinoma metastatic to lymph nodes simulating the "syncytial variant" of nodular sclerosing Hodgkin's disease. One of the patients initially was treated for Hodgkin's disease, but the clinical evolution was more typical of carcinoma. Both lesions were characterized histologically by noncohesive aggregates of large neoplastic cells with abundant eosinophilic cytoplasm and conspicuous nucleoli. Although cells compatible with diagnostic Reed-Sternberg cells were identified in an "appropriate" cellular background in both patients, the diagnosis of carcinoma was supported by intense cytokeratin immunoreactivity. Subtle histologic clues that should suggest the possibility of metastatic carcinoma in a patient whose morphologic data suggests the syncytial variant of nodular sclerosing Hodgkin's disease include sinus infiltration, phagocytosis of neutrophils by tumor cells, marked nuclear anaplasia, and the presence of spindle-shaped tumor cells.