We have performed constant temperature and pressure classical molecular dynamics simulations of crystals of the lecithin fragments glycerylphosphorylcholine (GPC), a model for the headgroup and glycerol regions, and dilauroylglycerol (DLG), a model for the glycerol/acyl ester and hydrocarbon regions. By comparing the simulation results to experimental X-ray crystal structures, we have evaluated the quality of all-atom force fields for phospholipids. For GPC we examined two existing force fields : the Stouch et al. (J. Comput. Chem. 1991, 12, 1033) and CHARMM 22 (MacKerell, A.; Schlenkrick, M.; Brickmann, J.; and Karplus, M. Unpublished) potentials. For DLG we employed a force field that is a combination of the Stouch et al. parameters for the glycerol/acyl ester linkage and a compilation of new and existing potentials for the hydrocarbon chains. The average unit cell parameters from the GPC simulation with the Stouch et al. potential were all within 2% of the experimental values, and the density was less than 4% too low. The agreement between the simulation and experimental cell parameters was better, and the density error was 3%, for DLG. The agreement of the CHARMM 22 model for GPC was not as good : the deviation of the unit cell lengths was as much as 7%, and the density was 11% too high. Overall, the molecular structures and hydrogen-bonding networks were well-reproduced by the DLG simulation and the GPC simulation employing the Stouch et al. potential. The most significant deviations were in the torsion angles in the acyl ester linkages of DLG, but the deviations in adjacent torsions offset each other, leaving the hydrocarbon chain packing unchanged. In general, the results of our simulations, in which the Ewald method was used to sum the electrostatic interactions, agree even better with the experimental data than the constant energy and volume simulations of Stouch et al., in which the electrostatic interactions were truncated.