Microdosimetric calculations of (131)I have been evaluated for a single cell and for cell clusters. A VsBasic program has been used to calculate stopping power, linear 3 energy transfer, range values and deposited energies per decay for beta particles, Auger and conversion electrons of (131)I. The chemical composition of the cell has been taken into account in this model; results were compared with water medium. Besides, total absorbed doses have been calculated for the radionuclides distributed randomly within the cell and clusters. Cross-fire irradiation has been considered for clusters of cells. In this case, absorbed doses per cell within a cluster were found to be significantly higher than absorbed doses per single cell, depending on the cluster size. Results showed that (131)I, is a promising radionuclide for therapy of tumors from millimeter to centimeter dimensions. (c) 2004 Published by Elsevier Ltd.