Early components of the auditory evoked response, arising in the brainstem, can be illicited by clicks or (coherent) tone bursts. Daly et al. [EEG Clin. Neurophysiol. (in press)] have shown that the latter, frequency-following response (FFR) represents the iterative activation of brainstem generators by succeeding cycles of the stimulus tone. In patients with acoustic neuromas, stimuli presented at less than some “critical off time” produce threshold instability. In four patients with acoustic neuroma, FFR at 500 Hz was markedly distorted or absent despite normal threshold sensitivity for the stimulus tone. At higher intensities, a response similar to the cochlear microphonic appeared. Such FFR's distinguish patients with angle tumors from patients with ochlear pathology, sensorineural loss, and intrinsic brainstem lesions. These studies offer a new methodology for evaluating the functional integrity of the VIII nerve and for early detection of acoustic neuromas. [Supported by N.I.N.C.D.S./NS02098.]