Abstract: The acoustic properties of so-called instant fit ear tips, which in contrast to otoplastics are not produced individually, should be able to be better taken into consideration when adjusting a hearing aid for an individual wearer. To this end a method is proposed in which an OLG measurement, in other words an open loop gain measurement, is performed on the hearing aid when it is being worn. The OLG measurement curve obtained is compared with an OLG reference curve. A value is ascertained from the comparison which represents the level of closure of the hearing aid or of its otoplastic in the ear of the wearer. On the basis of this value the gain of the hearing aid is finally corrected. Furthermore, a corresponding adjustment device is provided. It is thus possible in a simple manner to take into consideration the individual, current level of closure during the adjustment.