FERMI'S1 statistical thermodynamic theory of the generation of pions in collisions of high-energy nucleons has stimulated considerable discussion of the problem. The theory is in reasonably good agreement with observation ; but the theory is not free, as Fermi himself and others2 have pointed out, from conceptional difficulties. However, the purpose of the present note is to show that in the usual derivation1 for the number of pions produced on Fermi's theory, the contribution of the pions in the lowest energy-state is not adequately taken into account. In fact, because of the Lorentz-contraction of the volume (in which pions are produced) the usual estimate of the total number of pions produced has an appreciable error. In the absence of the Lorentz-contraction the error would be entirely negligible.