Comparison between pressure-volume relations in helium at high pressures indicates that theoretical calculations based on gas-phase pair potentials are not adequate to explain recent x-ray experiments. Regardless of what potential is used, the gas-phase potentials give an equation of state that is much too stiff. It is shown that if the changes in the atomic wave functions induced by the high-pressure crystal environment are taken into account, then agreement between experimental and theoretical pressure-volume curves is greatly improved.