The prevalence of childhood eczema is increasing in many countries. Epidemiological studies, however, say little of its association to outdoor air pollution and climate factors. We conducted a nationwide survey of middle-school students in Taiwan from 1995 to 1996. The 12-month prevalence of eczema was compared with air monitoring station data of temperature, relative humidity, and criteria air pollutants. A total of 317,926 children attended schools located within 2 km of 55 stations. Prevalence rates of recurrent eczema were 2.4 and 2.3% in boys and girls, respectively, with prevalence rates of flexural eczema 1.7% in both sexes. After adjustment for possible confounders, flexural eczema was found to be associated with traffic-related air pollutants, including nitrogen oxides and carbon monoxide. Recurrent eczema was associated with traffic-related air pollution only in girls. There were no associations for the highest monthly means of temperature, whereas the annual means and the lowest monthly means of temperature were negatively related to flexural eczema, but only in girls. The lowest monthly mean relative humidity was positively related to eczema. The results suggest that air pollution and climatic factors, which showed stronger associations in girls than boys, may affect the prevalence of childhood eczema.