The phase behavior of a system composed of spherical particles with a monomodal size distribution is investigated theoretically within the context of the van der Waals approximation for polydisperse fluids. It is shown how the binodals, spinodals, cloud-point and shadow curves as well as all the (polydispersity induced) critical points can be obtained for a variety of interaction potentials. The polydispersity induced modifications of the phase diagram (even for a polydispersity index I as small as I approximate to1.01) should be observable in some colloidal dispersions.