In recent years, concern has been raised about patient-specific doses due to the increased use of PET imaging. It is of scientific and practical interest for the accurate quantification of the doses to patients administered a radiopharmaceutical for PET imaging. In the present study, Monte Carlo simulations have been carried out to evaluate S values-the mean absorbed dose to the target region per unit cumulated activity in the source region-for positron emitters in the brain, heart and the urinary bladder content. The positron emitters considered were four radionuclides that are frequently used for PET imaging: (11)C, (13)N, (15)O and (18)F. S values were evaluated for the Medical Internal Radiation Dose 5 type mathematical phantom and the adult voxel phantoms developed at the Japan Atomic Energy Research Institute (JAERI). Consequently, Monte Carlo simulation and voxel phantoms were found to be useful in the evaluation of the beta dose to organs, in particular to hollow organs such as the urinary bladder. It was also demonstrated that the self-dose S values largely depend on the beta-ray energy and the mass of each target region.