Employing radionuclide scanning, the volume of the spleen, its red cell pool and plasma pool have been measured in vivo, and the relative proportions of cellularity and vascularity of the spleen have been calculated in 51 patients with myeloproliferative and lymphoproliferative disorders. In primary proliferative polycythaemia (polycythaemia vera), the increase of spleen size was attributed mainly to the increase of splenic vascularity; in myelofibrosis and in hairy cell leukaemia, the increase of spleen size was associated with increase in both splenic vascularity and cellularity, whilst in CGL and CLL the increase was attributed more to cellularity than to vascularity.