Tinnitus, the perception of noise in the absence of an external auditory stimulus, is common, frequently distressing and often intractable. It is associated with a number of conditions including deafness but may arise spontaneously. Brain imaging studies indicate increased neuronal excitability and decreased density of benzodiazepine receptors in temporal (auditory) cortex but the source and mechanism of such changes are unknown. Various electroencephalographic (EEG) abnormalities involving temporal lobe and other brain areas have been described but recordings have been limited to standard EEG wave bands up to frequencies of 22Hz. This clinical study of otherwise healthy patients with intractable unilateral tinnitus, using quantitative EEG power spectral mapping (QEEG), identified discrete localised unilateral foci of high frequency activity in the gamma range (>40-80Hz) over the auditory cortex in eight patients experiencing tinnitus during recording. These high frequency "hot spots" were not present in 25 subjects without tinnitus. The results suggest that further EEG investigations should include recordings in the gamma frequency range since such high frequency oscillations are believed to be necessary for perception. Identification of "hot spots" in tinnitus patients would provide a means for monitoring the effects of new treatments. These findings may also provide a model for exploration of more complex phenomena such as verbal and musical hallucinations.