Purpose: To assess the occurrence of two mutations associated with susceptibility to aminoglycoside ototoxicity.Methods: Genetic analysis of anonymized, residual diagnostic specimens. Results: One occurrence of the A1555G mutation and seven occurrences of the 961delT ϩ C(n) nucleotide change were found. Two previously unreported sequence changes, T961G and 956 -960insC, were also found in six and five specimens, respectively. Conclusions: Genetic susceptibility to aminoglycoside ototoxicity may be more common than previously suspected.Further study of the 961delT ϩ C(n) mutation is recommended to confirm its role in aminoglycoside ototoxicity and assess penetrance and variability with and without exposure to aminoglycoside antibiotics. Genet Med 2002:4(5):336 -345.