Abstract The effect of three-dimensional leakage on the extrapolated endpoint, Z 0 , is studied for non-multiplying, pulsed systems. The three-dimensional nature of the problem is treated accurately by modifying a method developed in an earlier publication on the diffusion length problem. Results are obtained using realistic scattering models for hydrogenous and polycrystalline moderators, and also in the 1-speed approximation, assuming isotropic scattering. The theory is compared with experimental values for particular systems. The variation of Z 0 with B 2 is demonstrated for cubical systems: Z 0 is found to decrease with increasing B 2 for hydrogenous and polycrystalline materials. This dependence is explained by considering the asymptotic spectra and the energy dependence of the neutron mean free path. Tables of Z 0 are included so that experimenters may deduce B 2 for non-cubic systems. The anisotropy of Z 0 is demonstrated by computing the value for each direction of a rectangular parallelepiped.