The German translation of the International Outcome Inventory for Hearing Aids (IOI-HA) was used in a variety of studies including a retrospective mailing campaign and three field tests on recently launched hearing aids. The psychometric properties were compared with each other and with results of previous studies in the U.S.A, Wales, and the Netherlands. Like the English and the Dutch version, the German IOI-HA showed high internal consistency, and factor analyses as well as inter-item correlations indicated the existence of a two-dimensional scale. Furthermore, the IOI turned out to be a valid instrument for distinguishing satisfied and non-satisfied customers during the trial period of a hearing aid.