Anaplastic large cell (Ki-1) lymphoma with histiocytic phenotype simulating carcinoma. Histiocytic and epithelial cell types share many cytomorphologic and functional characteristics; it is predictable, therefore, that corresponding malignancies might be difficult to distinguish. Described is the case of a 52-year-old woman in whom disseminated anaplastic large-cell lymphoma simulated carcinomatosis by conventional morphologic criteria. Evidence of histiocytic differentiation was derived from immunocytochemical, fine structural, and genetic probe analyses. The diagnosis of histiocytic neoplasia is discussed in relation to categories previously termed malignant histiocytosis, "Ki-1" lymphoma, and regressing atypical histiocytosis, and comparisons are made with animal tumor models. 