Within the last four years, clinical as well as engineering experience has been gathered with 12 deaf persons who received several versions of an auditory prosthesis. Two years ago we settled on a system consisting of a versatile, passive 4-channel implant driving a scala tympani electrode, and a small external speech-processor. The main characteristics of the prosthetic hearing which can thus be established are: dynamic ranges vary between 12 and 20 dB, the number of discriminable amplitude steps within the dynamic range is comparable to that of a normal-hearing person; subjective pitch increases with stimulation frequency up to at least 1,000 Hz. Thresholds and dynamic ranges as well as a number of other characteristics remain stable over long periods of time. After an initial big step to good open speech understanding by one of our patients it could now be shown, that this patient is not an exemption. Several deaf patients participating in a series of speech tests have demonstrated that it is possible to reestablish some understanding of open speech through the use of the cochlear prosthesis without additional lipreading. A program has been established which aims at achieving experience with this prosthesis in a larger number of deaf people. The program includes: implant candidate selection, implantation, adjustment of speech-processor, hearing and speech rehabilitation, and the collection of comparative" psychoelectric" characteristics.