Ten families in which there were 79 individuals affected by Waardenburg's syndrome were examined for penetrance of sensorineural hearing loss and expressivity of the gene. There were 47 with Waardenburg syndrome Type 1 and 32 with Waardenburg syndrome Type II. Penetrance of senorineural hearing loss was calculated after exclusion of the probands and was found not to be significantly different between each syndrome type but to show marked interfamilial variation. A bilateral sensorineural hearing loss was present more frequently than unilateral with the proportion varying between families. Certain audiometric shapes were found to recur in the syndrome but, apart from possibly one asymmetric configuration, seem to have been described also in other conditions. The degree of hearing loss was very variable within and between families. The implications for genetic counselling are discussed and the advantages of basing risk factors upon individual families rather than syndrome types emphasized.