The Gulf of Finland is one of the heaviest ship traffic areas in the world. The ice-covered period in the gulf lasts up to 140 days in severe winters. Common features are openings in ice (flaw leads) and ice ridges. The winter of 2002/2003 was exceptionally harsh: the entire Gulf of Finland was for a long time covered with thick ice, severe weather conditions caused much ice deformation and numerous ship incidents happened. We investigated the dependence of ice deformation rate on wind speed and direction, and variation in ice conditions in space and time. The numerical sea ice model HELMI was used to determine relations between wind conditions, ridged ice and ship damages. The occurrence frequency of leads in different regions was analysed by MODIS satellite imagery. The strongest wind blew from the NE, SW and NW in the winter of 2002/2003. The growth rate of deformed ice was more related to the wind direction than to speed. The ridging was most intensive when the wind blew from the E, SW and NW. Openings were formed almost everywhere during moderate or strong winds. Elongated leads, caused by northerly winds, were more common in the Finnish coastal region.