This investigation studied the electrical auditory brainstem response (EABR) inputoutput functions in three groups of patients: individuals with normal auditory neural systems; patients with acquired, profound sensorineural hearing loss; and patients with congenital, profound sensorineural hearing loss. Each input-output function was obtained in the operating room under general anesthesia and with the patient paralyzed to eliminate myogenic contamination. In contrastto the acoustically elicited ABR, the EABR waveforms were characterized by a dominant early wave corresponding in latency to wave II. This wave was much larger and often appeared at lower stimulus intensities than wave V. The results, plotted according to rate of growth for waves II and V, showed that normal subjects generally had larger, more robust early waves than either hearing impaired group. Little difference was noted in the amplitudes or rates of growth for wave V among the three groups. Based on these observations, we concluded that the EABR demonstrated differences in the way normal and impaired systems respond to electrical stimulation. In addition, the EABR may be useful in providing a qualitative prediction of neural survival into broad categories of many surviving neurons versus a marked reduction in ganglion/neuronal density.