This study compares amplitude-modulation (AM) masking in listeners with normal hearing and in listeners with a hearing loss. To address this issue, we measured the detection of sinusoidal AM applied to a white noise carrier, as a function of the frequency of a masking sinusoidal AM applied to the same noise carrier. These input filter patterns were measured for four listeners with normal hearing and three listeners with moderate or mild-to-severe sensorineural hearing losses. Stimuli were presented at 50 dB SL for all listeners with normal hearing and for two of the three listeners with hearing loss. The third listener with hearing loss was tested at 25 dB SL. For the listeners with normal hearing, the input filter patterns obtained for 100-Hz signal modulation had a broad bandpass characteristic. All input filter patterns showed a primary masking peak at 100 Hz. A secondary masking peak was apparent also at 50 Hz. For the listeners with impaired hearing, the unmasked modulation thresholds were similar to those measured in the listeners with normal hearing. One listener with a moderate hearing loss exhibited a broadly tuned input filter pattern with a masking peak at 100 Hz, but no secondary peak. The two other listeners with moderate or mild-to-severe sensorineural hearing loss showed no main masking peak and increased thresholds at low masker modulation frequencies. These results suggest that cochlear damage may affect performance in a modulation masking task.