The purpose of this study was to compare hearing and understanding with a bone conduction hearing aid of a new type and with conventional models. The new instrument, BAHA (bone anchored hearing aid) is connected with the skull percutaneously rather than transcutaneously as with the conventional instruments. In the ENT department of Nijmegen University Hospital the understanding of speech with both types of hearing aid was evaluated thoroughly. The patients had a middle-ear loss with in addition an inner-ear loss of 60 dBHL at most. A conventional air conduction hearing aid (behind or inside the ear) was unsuitable for them because of, for instance, chronic runny ears or anomalies of the auditory meatus. A conventional bone conduction hearing aid containing a transducer pressing on the skin was rejected because of poor understanding or serious side effects such as headache and pressure pains. At the first session a titanium screw was implanted in the skull behind the ear. The percutaneous superstructure was put into place a few months later at a second session. In none of the patients were there peroperative problems or postoperative infections of any importance. Understanding of speech in silence and in noise was evaluated with the BAHA and with the conventional aid. Differences in understanding of speech were regarded as significant if they amounted to more than twice the known intra-individual standard deviation. As to understanding of speech in silence, 7 of the 15 patients were found to understand significantly better with the BAHA than with a conventional aid. In the other patients no difference was found.(ABSTRACT TRUNCATED AT 250 WORDS)