Although a large number of fitting procedures have been developed and are nowadays generally applied in modern hearing aid fitting technology, little is known about their effectiveness in comparison with each other. This paper argues the need for comparative validation studies on hearing aid fitting procedures based on the design of a randomized clinical trial and carried out in a large-scale clinical population. These studies are hard to conduct but can provide detailed information on the various aspects of the rehabilitation with hearing aids. The design of several recently reported comparative studies of hearing aid fitting procedures will be reviewed. This gives rise to a number of comments on aspects like, study design, composition of the study population and definition of outcome measures rather than on the outcome or conclusions of these studies themselves.