It is shown that the most suitable boundary conditions connecting the moments of the angular flux at the two faces of an annular void surrounding the fuel in a reactor lattice cell are obtained from the consistent use of the transport-theory PN approximation in the void as well as in the other media, when N is not restricted to very low values. A different procedure, which is particularly appropriate in diffusion theory, had been used previously by Newmarch and extended to N = 3 and N = 5 by Tait and Clendenin. However, the algebraic difficulties are considerable, and, although their method is preferable to the systematic use of the PN approxi- mation in all media for the lowest values of N, it is not capable of generalization to higher N. Comparisons of the different approaches are given for the lowest-order approximations; the method based on applying the PN approximation to all regions is given in a form suitable for any odd value of N, and numerical results are presented up to N = 11 to show that it converges rapidly.