The ability of trajectory models and meteorological data bases to simulate long-distance transport of air pollutants is assessed by comparing trajectories of tetroons and tetroon grounding locations from several recent U.S. Environmental Protection field studies with diagnostic trajectories computed from National Weather Service (NWS) wind measurements. Trajectories from three different models were used in the assessment; trajectories based on 24-h surface wind data, isentropic trajectories based on 12-h NWS radiosonde data, and boundary layer trajectories based on 6- and 12-h NWS radiosonde data. The results suggest that a large uncertainty is inherent in diagnostic trajectories regardless of the model or wind data used.