The authors undertook a cross-sectional study of the potential adverse health effects of air pollution in Bangkok, Thailand. During 1998 and 1999, the authors administered lung function spirometry tests and a Thai version of the American Thoracic Society's Division of Lung Diseases (ATS-DLD) respiratory questionnaire to 78 male traffic police and 60 male nontraffic police in Bangkok, as well as to 68 male general police in Ayutthaya province, a rural area in Thailand. No consistent trend of decreased pulmonary function was observed in traffic police. The authors controlled for age, height, and smoking index, after which mean levels of forced expiratory volume in 1 sec and maximal expiratory flow rate in 25% of vital capacity (V25) were significantly lower in Bangkok police than in Ayutthaya police. The prevalence of respiratory symptoms among Bangkok police was slightly higher than among Ayutthaya police. Multiple regression analysis identified age and workplace as statistically significant factors that contributed to the values of forced expiratory volume in 1 sec and V25. This study provided some evidence of an increase in prevalence of obstructive changes in the peripheral airways among traffic police in Bangkok.