An extensive survey has been carried out in 52 underground car parks in Hong Kong focusing on carbon monoxide concentrations in the air. So far as shortterm health effects are concerned this compound is generally regarded as the most important chemical species from car exhausts. Its presence can be used to quantify air pollution, and give an indicator of the air quality in car parks. The sites were sampled during peak hours. This report presents the results and analysis of the carbon monoxide levels measured. Of the 52 sites surveyed, 27 were rated as ‘good’, 7 ‘acceptable’ and 18 ‘poor’ according to our interpretation of recommended international health criteria. The findings also revealed that the utilisation rate of car parks varied significantly over time.