The results of the third year of observations on the effects of self-irradiation on the lattice of 238(80%)PuO2 indicate a small but definite trend toward a reduction in the lattice parameter of this material. The lattice parameter increased from 5.41408±6 Å at 5 weeks to 5.41636±38 at 114 weeks and then decreased to 5.41364±23 at 157 weeks after the start of the experiment. A similar trend is noted in the half-widths of the diffraction lines. For the 622 reflection the half-width increased from 0.283±14°20 at 5 weeks to 0.711±90 at 114 weeks and then decreased to 0.660±89 at 157 weeks.