The voxel type phantoms BABY and CHILD are used to quantify the dependence of organ dose per exit dose conversion factors on patient diameter in diagnostic paediatric radiology by variation of the voxel size in the relevant dimension. The results of Monte Carlo simulations of the five most frequent examinations, i.e. skull AP, thorax AP and PA, abdomen AP and pelvis AP, are presented for both phantoms. The organ doses increase very differently with increasing patient diameter, depending on the phantom, the examination, the radiation quality and the relative position and percentage of the organ in the beam. Using these results, phantom and examination specific organ dose scaling factors from 'standard patients' to patients with different diameters can be obtained.