In minimal supergravity theories the soft supersymmetry breaking (SSB) parameters are universal near the Planck scale. Nevertheless, one often assumes universality at the grand-unification scale MG≈1016 GeV, and corrections to the SSB parameters arising from their evolution between the Planck and grand-unification scales are neglected. We study these corrections in minimal SU(5) and show that large splittings between the scalar mass parameters can be induced at MG. These effects are model dependent and lead to significant uncertainties in the predictions of supersymmetric models.