A wide variety of human neoplasms were examined by immunocytochemical and ultrastructural techniques. In most, one intermediate filament (IF) type was expressed reflecting the tissue of origin. However, multiple classes of intermediate filaments were regularly found in a subgroup of these tumors. We chose to subdivide them into those with a complex or mixed growth pattern, and those which showed a more "monomorphic" histologic growth pattern. This latter group is the subject of this paper. Regular coexpression of cytokeratin and vimentin was observed in tumors of endometrial, thyroid, ovarian and renal origin, and coexpression of cytokeratin and neurofilament was observed in a subgroup of neuroendocrine tumors. Immunocytochemical/ultrastructural correlation demonstrated few, if any, observable intermediate filaments in tumors expressing only low molecular weight cytokeratin, whereas vimentin and neural filament characteristically were randomly dispersed or formed whorled bundles of cytoplasmic filaments. The potential diagnostic usefulness of these observations in surgical pathology is discussed.