An investigation of the status of electron equilibrium at the surface of a lesion located in an air cavity of the upper respiratory system irradiated with cobalt 60 teletherapy has been carried out. The experimental method consisted of measuring the “build-up” of ionization in a small ionization chamber in a masonite phantom. This phantom had an air volume in the shape of a rectangular parallelepiped constructed so that its dimensions could be varied to cover the anatomical range. The photon beam passed through this air volume before reaching the ionization chamber. The departure of the reciprocal of the ionization build-up ratio (BUR)−1 from unity was measured as the dimensions of the irradiating field and of the parallelepiped were varied. For practical cases encountered in radiation therapy this departure is not greater than 10 per cent, but should be accounted for if allowances for increased beam penetration through air are made.