A terrestrial laser scanner was used to measure the three-dimensional structure of forest stands in the Swiss National Park, eastern Switzerland. Spatially coincident hemispherical photographs were taken at each sampling point and the position of each point was determined using differential GPS. A scanner model was derived in order to determine the expected number of laser shots in all directions, and these data were compared with the measured number of laser hits to determine directional gap fraction at nine sampling points. Directional gap fraction distributions were determined from the digital hemispherical photography and compared with distributions computed from the laser scanner data. The results showed that the measured directional gap fraction distributions were similar for both hemispherical photography and terrestrial laser scanner data with a high degree of precision in the area of overlap of orthogonal laser scans. Unlike hemispherical photography the laser scanner data offer semi-automatic measurement of gap fraction distributions, plus additional three-dimensional information about tree height, gap size distributions and foliage distributions.