A hearing aid fitted to different ears will produce very different sound pressure spectra in the ear canal. In addition, this variation in response is different among hearing aids. A description in terms of an electrical analog model of the ear and hearing aid system is given. The applicability of this model is tested through series of measurements. The measurement and prediction procedure was first verified on a coupler (ear simulator) with good results from 300 to 8000 Hz. Three types of hearing aids were then measured and used on five different human ears. Where the measured and predicted response was compared a fairly good agreement was obtained from 300 Hz to approximately 6000 Hz. A major source of error is probe misalignment. The theoretical description given in the present work is likely to be valid from low frequencies to at least 10 kHz. Although solutions for related problems have been given for low frequencies, a solution of the hearing aid fitting problem at high frequencies has not been published earlier.