Two-dimensional elastic stress field behaviour near the mouth of a surface crack in an isotropic half-plane is examined. It is shown that a dislocation density simulating the crack opening and consequently the stress field has a singularity at the point in question. The type of singularity is determined by the distribution of the load applied as well as by the angle between the crack contour and the surface. Two known solutions for stress intensity factors of a uniformly loaded crack are compared, the first taking the singularity into account and the second neglecting it. It is found that the solutions differ significantly when the crack is inclined at an acute angle.