The subjective symptom of hyperacusis is described. The terms 'hyperacusis' and 'phonophobia' are considered to be synonymous as there is no recognized distinction between these two descriptions. Peripheral auditory pathologies with associated hearing sensitivity are reviewed and the likely mechanisms underlying the hyperacusis are listed. The neurological conditions, which have been reported to occur with hyperacusis, are reviewed. A separate aetiology of central hyperacusis is therefore proposed, with a symptom profile distinct from the peripheral hyperacusis. A common factor to neurological conditions with hyperacusis, is disturbance of 5-HT (5-hydroxytryptamine) or serotonin function. The research literature on the role of 5-HT in sensory modulation (specifically auditory startle) in animals is presented. It is proposed that 5-HT dysfunction is a probable cause of increased auditory sensitivity manifested as central hyperacusis or phonophobia.