This article reports the findings of a pilot field study carried out over a one-week period in winter 1996 to investigate the internal and external air pollution levels of two adjacent buildings, one naturally ventilated and the other air-conditioned in an urban area, to investigate their relative attenuation of external pollution levels, and to compare internal levels with existing air quality guidelines. Ventilation in the naturally ventilated building is not through design strategy, but simply through openable windows. Concentration levels of sulphur dioxide, nitrogen oxides, carbon monoxide and carbon dioxide were monitored. Simultaneously, measurements of ventilation rates within the buildings and periodic video recordings of the traffic were also carried out. As expected, the concentrations of external pollutants in the buildings followed the daily external variation, but at reduced levels. Generally, pollutant levels were higher in the naturally ventilated building than in the air-conditioned building. However, on a number of occasions, combustion products from heating boilers were entrained into the air-conditioned building via the high-level air intake of the ventilation system, raising the levels of nitrogen oxides, sulphur dioxide and carbon dioxide inside the building to higher than those found externally. A comparison of the results with existing air quality guidelines or standards for exposure showed that, in both buildings, the level of contamination was less than the relevant standard, except during a limited period at the weekend when combustion products were possibly entrained into the air-conditioned building. Based on this short pilot study, there appears to be no clear distinction between the two types of ventilation strategies in terms of providing adequate indoor air quality to the occupants of the buildings.