One of the routine ground tests carried out on pressurised aircraft is the measurement of the leakage rate and this is often expressed in the form of a time for the cabin excess pressure to fall from a given value to half that value. The technician, however, is usually concerned with the functioning of the pressurised cabin at high altitude and it is the purpose of this note to establish a means of forecasting the high altitude performance from the sea level test. The first part of the note describes the basic theory and provides a unique relationship connecting leakage times at any height with that pertaining to the sea level test. The latter part of the note gives an example of the use of this information in predicting the fall off of cabin pressure after pressurisation failure and with the aircraft descending.