The paper describes an investigation of short-term effects of NO2 concentrations in the air on the number of emergency room visits caused by respiratory impairments, particularly asthma in adults and children. The data were collected from clinical emergency room records from July 1, 1994 to December 31, 1995. Concurrently, readings of average weekly concentrations of NO2 (microgram/m3), average weekly temperature (degree C), air pressure (kPa), and relative humidity (%) were registered. Trend and seasonality effects were estimated by the locally weighted regression (LOESS). After standardising for trend, seasonality, and meteorological conditions, the number of cases was regressed on weekly NO2 concentration, including the current and the previous week concentrations and autocorrelated residual. The weekly average NO2 concentrations were significantly associated with the number of emergency asthma cases for children and adults and with the total number of emergency respiratory cases in children, but not in adults. The results suggest that health effects of NO2 on risk groups can be detected even in moderately polluted environments. The effect is more pronounced in children.