Sensorineural deafness associated with increased sensitivity to aminoglycoside antibiotics as the consequence of an A1555G mutation in the mitochondrial DNA (mtDNA) in a highly conserved region of the small (12S) rRNA gene has been reported in Caucasian, Chinese, and Japanese individuals. We report here a large family of Balinese Indonesian origin with progressive/congenital sensorineural deafness who carry the A1555G mutation. The pedigree shows a generally maternal inheritance pattern with some exceptions, which is the result of an unusual multiple entry of the mutation into the pedigree. A complete mtDNA genome sequence from three Balinese individuals revealed a relatively large number of single- nucleotide polymorphisms (20) not previously reported, and confirmed the genetic distance of Southeast Asian populations from those of Caucasians and Japanese. The biochemical expression of the A1555G mutation under the influence of this mtDNA background was investigated. Examination of respiratory enzyme activities showed a significant decrease in respiratory complex I activity, particularly in symptomatic family members.