Ionization of atoms by proton impact, as predicted by the classical binary-encounter approximation, is examined and compared with available experimental data on the noble-gas and alkali-metal atoms. The results indicate that these predictions agree with observation to within a factor of 2 or 3, and are as reliable as the comparable results for electron ionization. Comparisons are made between electron and proton ionization cross sections. The proton ionization curves all have a single maximum and are otherwise monotonic, in contrast to electron ionization, where the effects of atomic shell structure produce secondary maxima for some atoms. The effect of averaging over bound-state speed distributions is discussed, and is found to be much less important for protons than for electrons, except at very low proton energies. The need for experimental measurements at higher energies (\ensuremath{\sim}10 MeV) is discussed.