Studies with transmission and scanning electron microscopes (TEM and SEM) were performed on specimens from patients suspected of leukemic reticuloendotheliosis (LRE), irrespective of the final diagnoses. The blood samples were fixed immediately in order to avoid in vitro changes of leukocytes. Leukemic reticuloendotheliosis cells demonstrated numerous long microvilli and pseudopods, whereas the immediately fixed lymphocytes demonstrated fewer, shorter microvilli and no pseudopods. These surface features distinguishing LRE cells from lymphocytes and monocytes were appreciated by both SEM and TEM. In addition, TEM revealed significant differences in the incidence of certain ultrastructural alterations. Whereas nuclear pockets were rare in lre cells, they were relatively frequent in lymphocytes of other hematologic disorders. The inclusions of ribosome-lamella complexes were not only more prevalent in patients with LRE but were seen in a higher percentage of leukemic cells from patients with LRE than in those from patients with other hematologic disorders. Electron microscopy can play a role in the laboratory diagnosis of LRE.