OBJECTIVES: To determine the effect of spectral differences between stimuli used for verification of hearing aid gain in nonlinear devices. DESIGN: The spectra of nine stimuli from five analyzing systems were obtained. RESULTS: Most stimuli closely duplicated one of two well-specified spectra. The difference between these two input spectra resulted in measured gain differences of up to 8 dB. CONCLUSIONS: Careful consideration of selected test stimulus for measuring hearing aid gain is recommended.