Three hearing aid manufacturers provided custom full-shell in-the-ear hearing aids for each of 3 hearing-impaired subjects. Each manufacturer was instructed that the hearing aids should provide the maximum possible acoustic gain within the limits of hearing aid shell size and available components. Coupler gain, insertion gain, and functional gain were measured for each hearing aid. Gain measures were made with the volume control at either the full-on setting or the highest setting possible before the onset of acoustical feedback. Full-on coupler gain curves were similar across all nine hearing aids. Individual differences in concha/ear canal size and in the fit of the hearing aids produced substantial variance in insertion gain across hearing aids. Peak insertion gain varied from 41 to 58 dB. If 10 dB reserve gain is allowed, the range of estimated peak use gain from these maximum-gain in-the-ear hearing aids is 31-48 dB.