In a method of fitting a hearing aid system, a hearing aid user's hearing loss is classified and the hearing aid fitting is adapted in response to this classification. The hearing loss classification is, in one example, classified by determining an audiogram for the user, testing the user's hearing function under conditions that, for a user with that audiogram, would result in a predicted intelligibility, and comparing the measured intelligibility to the predicted intelligibility. Also disclosed is a hearing aid fitting system (100, 200) adapted to carry out the method.