Lung dose correction factors, commonly defined as the ratios of ionization chamber readings in the heterogeneous and homogeneous phantoms, have been compared with those based on accurately determined doses. An analysis of stopping power values, Pwall values, and measurements in lunglike and waterlike materials showed that the wall material and thickness are not very critical in the determination of lung dose correction factors under conditions of electronic equilibrium. When lateral electronic equilibrium is not established due to the extended range of scattered electrons in the low density material, Prepl differs significantly from unity for ionization chambers with thick walls which do not match the lung material in density. An attempt has been made to characterize this effect as a function of photon energy, lung density, field size, and wall thickness.