A great interest is being taken in the measurement of concrete permeability because of increased concerns about the durability of concrete when exposed to aggressive environments. However, the variation of the concrete permeability with time has not been systematically studied. Air permeability results recorded for 20 years in six 0.2 m(3) concrete slabs are presented. These results have a qualitative value as boundary effects are unknown. The great difference in flow between the specimens with the same mix design, casting and curing procedure, which have been kept in the same testing room sheltered from rain, is noticeable. The air permeability coefficient increases with time, reaching an almost stable value after 20 years, The pore evaporated water creates a path for air flow through the concrete and this is supposed to be the main cause of the increase in permeability. The method presented in this paper gives reliable information about the quality of the concrete studied in terms of air permeability and hence, durability.