A series of AlN films of increasing thickness (up to 4 mum) were grown on c-plane sapphire by metalorganic vapour phase epitaxy. Plan-view transmission electron microscopy (TEM) images reveal that the dislocation density at the film surface reduces with increasing film thickness, whereas cross-sectional TEM data reveal that dislocation reduction continues to occur beneath the film surface during growth, resulting in the preferential annihilation of c-type and/or (a+c)-type dislocations. We conclude that dislocation movement occurs by climb during AlN growth.