The leaf inclination-azimuth angle distribution is a major determinant of the electromagnetic response of vegetation canopies. In field and modelling studies, researchers commonly assume that (i) leaf orientation can be described by two separate distributions of inclination and azimuth angles, (ii) the leaf inclination angle distribution of a particular vegetation type and growth stage is diurnally static and (iii) the leaf azimuth angle distribution is symmetric. In this study the three-dimensional orientation of leaves described as a leaf inclination-azimuth angle distribution plotted in polar co-ordinates was measured throughout a day for a cotton and a soybean canopy. The results and the literature review showed that this distribution can vary significantly on a diurnal basis due to vegetation type, heliotropic leaf movement, environmental conditions (e.g. wind) and vegetation stress (e.g. water stress). The study also showed that it is erroneous to treat two separate distributions of azimuth and inclination angles rather than one three-dimensional distribution of leaf orientation. The three-dimensional distribution needs to be routinely collected in diurnal sun angle studies and incorporated into mathematical models of sensor response.