In two cases of hairy-cell leukemia were studied the transmission and scanning electron microscopy morphology, the cell surface markers with monoclonal antibodies and the response to polyclonal mitogens, the clinical course and staging. We have found in the hairy cells of both cases a characteristic membranous system consisting of several parallel cisternae arranged in series at regular intervals each bounded by a smooth membrane and interrupted at intervals by electron-dense structures: this structure can be identified with the 'annulate lamellae', as yet reported in many cells but never described in hairy cells. The results of immunological investigations are presented and discussed. Our data seem to support the neoplastic B lymphocyte nature of the hairy cells. Some interesting findings of the clinical course of the two patients are also stressed.