The many applications of Monte Carlo modeling in nuclear medicine imaging make it desirable to increase the accuracy and computational speed of Monte Carlo codes. The accuracy of Monte Carlo simulations strongly depends on the accuracy in the probability functions and, thus, on the cross-section libraries used for photon-transport calculations. A comparison between different photon cross-section libraries and parameterizations implemented in Monte Carlo simulation packages developed for positron emission tomography and the most recent Evaluated Photon Data Library (EPDL97) developed by the Lawrence Livermore National Laboratory, Livermore, CA, was performed for several human tissues and common detector materials for energies from 1 keV to 1 MeV. Different photon cross-section libraries and parameterizations show quite large variations when compared to the EPDL97 coefficients. This latter library is the more up-to-date complete and consistent library available, and was carefully designed in the form of look-up tables providing efficient data storage, access, and management. EPDL97 is already a standard in the nuclear reactor industry. Its use as a standard in the simulation of medical imaging systems will help to eliminate potential differences between the results obtained with different codes. Together with the optimization of the computing time performances of the Monte Carlo software package, Eidolon, photon transport in three-dimensional (3-D) positron emission tomography could be efficiently modeled to develop accurate scatter models and better understand scatter correction techniques.