This article compares mean wind estimates from a WAsP analysis for three forest sites and one site near a forest with measurements taken at the sites. By standard WAsP settings for forest, the mean wind speed at the sites was overestimated. Agreement between the estimates and the measurements improved significantly if displacement height and roughness length as calculated from the forest mast data were used or if a simple model estimate of roughness length and displacement height based on stand density (frontal area index) was used. The two estimates of displacement height and roughness length (mast data and simple model) did not agree well with each other. One reason for this may be that all evaluated sites are windy and that both d and z0 depend on the wind speed. All analysed forest sites are dense, in which case the influence from the roughness sublayer is limited and the effect on mean wind speeds from this layer could not be evaluated. Copyright © 2005 John Wiley & Sons, Ltd.