We report a study on the unfolding behavior of the most abundant protein contained in plasma, the fatted and defatted human serum albumin, in denaturing conditions induced by guanidine hydrochloride. Low-resolution three-dimensional structures are reconstructed from the one-dimensional (1D) small-angle X-ray scattering patterns, and they are correlated with the parameters obtained from static and dynamic light scattering experiments. The unfolding process is pointed out by both ab initio and rigid body fitting methods which highlight a stepwise evolution of the protein structure toward open conformations. The superpositions of the 3D structures provided independently by the two methods show very good agreements. The hydrodynamic radii estimated for the protein best fitting conformations are in satisfactory agreement with the experimental ones. The results show that the unfolding process is consistent with previous spectroscopic studies which suggest a multistep unfolding pathway. In particular, a scheme in which domains III and II are opened in sequence and the presence of two intermediates are evidenced is presented.