The binding and total exciton energies in zincblende and wurtzite GaN/Al0.2Ga0.8N quantum wells are calculated taking into account the existence of graded interfaces. It is shown that a strong exciton energy blue shift is due to the nonabrupt interfaces. In the case of the wurtzite phase, a,graded interface I nm thick blue shifts by approximately 200 meV the exciton total energy practically independent of the well width. In contrast, in the zincblende phase the blue shift depends strongly on the well width due to the absence of built in electric fields related to the piezoelectric and spontaneous polarizations. (C) 2002 Elsevier Science B.V. All rights reserved.