Analysis of the forced expiratory spirogram using time as the primary variable had been shown to be a promising method for detecting small airway dysfunction. This paper describes its application on 277 fire-fighters. Spirometry was performed when the fire-fighters were at rest and had not been fighting fires for the previous 24 hours. The best of three spirograms were used to compute the mean transit time (MTT). Through multiple linear regression analysis, age was the only factor that was significantly associated with MTT. The regression coefficients for age of smoking and non-smoking fire-fighters were nearly equal (0.011 and 0.010 seconds per year respectively). When 28 pairs of smoking and non-smoking fire-fighters were matched for age, duration of service, ethnicity and job groups, their average MTT were not significantly different. An increasing age-adjusted mean MTT was seen among the non-smoking fire-fighters with job groups requiring greater exposure to fire-fighting. Normal smokers had been shown to have a much greater regression coefficient for age on MTT when compared to normal nonsmokers. When compared with this study, the regression equation for MTT of non-smoking fire-fighters differed markedly from that of normal non-smokers and approached that of normal smokers. Coupled with the 'dose-response' relationship shown in the non-smoking fire-fighters, this study provides evidence of pulmonary dysfunction in fire-fighters. This pulmonary dysfunction may be localised in the small airways.