The study examines the influence of naturally elevated ozone concentrations and some meteorological variables on the lung function of untrained volunteers walking in an unpolluted mountain area. Forty male participants between 18 and 70 years (smokers and nonsmokers) walked at the top of Medvednica Mountain near Zagreb, Croatia, at approximately 1000 m above sea level (a.s.l.) and engaged in other recreational activities for at least 1 hour. Forced vital capacity (FVC) and forced expiratory volume in 1 second (FEV(1)) of the participants were measured at arrival and before they left the mountain. Regression analysis showed a statistically significant influence of environmental variables on lung function tests. However, all variables together explained less than 35% variability of FVC and 41% variability of FEV(1). The results suggest that short-term exposure to ozone may affect lung function tests, but no more than temperature and humidity. FEV(1) was found to be more sensitive to ozone than FVC. Smoking habit also had an important role in subject's sensitivity to ozone.