Several point mutations in mitochondrial DNA (mtDNA) have been shown to cause sensorineural hearing impairment (SNHI), but the frequency of these mutations among patients is not known. We identified 117 patients with possible matrilineal SNHI from population-based registers and found the 3243A > G mutation to be present in 4.3% and 1555A > G in 2.6%, while 7445T > C, 7472insC and 8344A > G were absent. Patients with 3243A > G and 1555A > G were clinically distinct. The prevalence of 1555A > G in the general adult population was estimated to be at least 4.7/100,000, but these and previous data suggest that the figure may vary between populations. Screening for mtDNA mutations is worthwhile in connection with the diagnosis of SNHI.