Noise and distortion reduce the sound quality in hearing aids, but there is no established procedure for calculating sound quality in these devices. This presentation introduces a new intelligibility and sound-quality calculation procedure based on the Speech Intelligibility Index [ANSI S3.5-1997]. The SII involves measuring the signal-to-noise ratio (SNR) in separate frequency bands, modifying the estimated noise levels to include auditory masking, and computing a weighted sum across frequency of the modified SNR values. In the new procedure, the estimated signal and noise levels are replaced with estimates based on the coherence between the input and output signals of the system under test. Coherence is unaffected by linear transformations of the input signal, but is reduced by nonlinear effects such as additive noise and distortion; the SII calculation is therefore modified to include nonlinear distortion as well as additive noise. For additive noise, the coherence calculation gives SII scores identical to those computed using the standard procedure. Experiments with normal-hearing listeners using additive noise, peak-clipping distortion, and center-clipping distortion are then used to relate the computed coherence SII scores with the subjects’ intelligibility and quality ratings. [Work supported by GN ReSound (JMK) and the Whitaker Foundation (KHA).]