The present work is concerned with integrability properties of derivatives of classical solutions of Dirichlet's problem for a linear second-order elliptic equation Lu = f. With the aid of special weighted Hilbert spaces of locally square integrable functions, we determine the nature of singularities that f can have near 1 the boundary, in order that such classical solutions are in the Sobolev space W. By means of an example it is shown that the obtained result is exact.