The Polar Vortex Balloon Experiment (POVORBEX) has flown eight long-duration flights 1992 to 1995 in the arctic winter stratosphere at about 50 mbar. European Centre for Medium-Range Weather Forecasts (ECMWF) analyses are compared with the balloon temperatures and winds, which are accurate and independent, and substantial differences are found. For example, the average temperature difference is 2.4 K. These temperature differences have important implications for the potential for polar stratospheric clouds, which play a vital role in the destruction of ozone. Large differences are also found between the observed and calculated trajectories. For the longest flight, which lasted almost 6 days, the calculated trajectory end point is off by 2700 km (23% of the trajectory length). For the other seven POVORBEX flights, which lasted-about 1/2--3 days, the calculated end point is off by 156--544 km (5--19% of the trajectory length for six of the flights). The main causes of these differences are discrepancies between real and ECMWF analyzed winds and temperatures and the large distance between the model levels at this altitude. ¿ American Geophysical Union 1996