This research examines the nature of acoustic-phonetic impairments found in aphasia, and the reliability of patient performance on phoneme discrimination and identification tasks. Aphasic patients were tested on three phoneme discrimination tasks examining their ability to discriminate items on the basis of contrasts in sonorance, manner, place, or voicing using both spoken and synthetic stimuli. One of these tests involving the discrimination of spoken, one-syllable items was given to patients on three occasions over the course of 1 year to examine short- and long-term test reliability. In addition to these measures, patients were tested on three phoneme identification tasks using stimulus items drawn from the discrimination tests. The results of these tests show that aphasic patients may display stable patterns of performance on these measures over time and between tests. Furthermore, these findings suggest that discriminations of synthetic speech do not necessarily reflect patients' ability to discriminate spoken speech tokens. These results are interpreted as evidence that aphasics display stable, discrete impairments in acoustic-phonetic processing, and that these deficits may be measured reliably using phoneme discrimination and identification tasks using natural speech.