Relative windspeed reduction was measured behind nine relatively narrow, homogeneous windbreaks in southern Ontario, Canada to assess whether any characteristics of the windspeed reduction curve could be predicted from optical porosity. The latter was determined for each windbreak using high contrast black and white photographic silhouettes on a computer digitizing system. Minimum windspeeds behind the windbreaks ranged from 29 to 71% of open windspeed; these minima were located 2 to 6 multiples of windbreak height away from the windbreak. Optical porosities of the bottom half of the windbreak ranged from 0 to 31%. Multiple regression of the shelter parameters (location and value of minimum relative windspeed) on the independent variables (optical porosity, open windspeed, surface roughness, approaching wind direction relative to the windbreak, average tree diameter and average tree spacing) showed that the minimum relative windspeed could be predicted from the optical porosity of the bottom half of the windbreak. The results suggest that optical porosity can be used to predict minimum relative windspeeds and may therefore be useful as a guide in the field evaluation of windbreaks.