High external noise levels are often used to justify the use of air conditioning in commercial and residential buildings. Methods of estimating noise levels in urban canyons are necessary if the potential for naturally ventilating buildings is to be assessed. A series of noise measurements were made in 'canyon' streets in Athens with aspect ratio (height/width) varying from 1.0 to 5.0. The main purpose of the measurements was to examine the vertical variation in noise in the canyons in order to give advice on natural ventilation potential. A simple model of the noise level has been developed using a linear regression analysis of the measured data. The model can be used to predict the fall-off (attenuation) of the noise level with height above street level. The attenuation is found to be a function of street width and height above the street, but the maximum level of attenuation (at the top of the canyon) is almost entirely a function of the aspect ratio except in narrow streets. Background noise (L-90) suffers less attenuation with height than foreground noise (L-10). Measurements of acoustic comfort in a survey throughout Europe are used to estimate the potential for natural ventilation in canyon streets. (C) 2004 Elsevier B.V. All rights reserved.