Ground level ozone arises primarily from traffic, it is a powerful oxidant and its primary target organ is the lung. Most epidemiological studies reporting the health effects of ozone have estimated individual exposure from measurements obtained from outdoor monitors but surrogates of personal exposure may not adequately reflect personal exposures. Also, the main focus has been on infants and children. Our purpose was to assess associations between urban background ozone and indoor residential ozone levels as well as to investigate the effects of indoor residential ozone on lung function in 51 elderly non-smokers. Indoor ozone was measured passively in homes, while urban background outdoor ozone was monitored continuously at a fixed monitoring station located on the roof of the 20-m high university H.C. Ørsteds campus building in a park area. Lung function was measured at baseline as well as on three consecutive occasions, for each subject. The mean residential ozone levels were 1.33 ppb, and mean outdoor urban background levels were 27 ppb. Outdoor urban background ozone levels were not consistently associated with residential ozone. No significant changes in lung function were detected in association with residential ozone among healthy participants. In this study, we were unable to detect significant changes in lung function in association with increased levels of residential ozone amongst healthy elderly non-smokers.