Congenital deafness occurs in approximately 1 in 1000 live births. In developed countries about 60% of hearing loss is genetic. However, in Brazil most cases of hearing loss are due to environmental factors, such as congenital infections (mainly rubella), perinatal anoxia, kernicterus and meningitis. Recently, it has been demonstrated that the GJB2 gene is a major gene underlying congenital sensorial deafness. Mutations in this gene cause 10-20% of all genetic sensory hearing loss. One specific mutation, 35delG, accounts for the majority of mutant alleles. The extent of the hearing impairment varies from mild/moderate to profound, even within the patients homozygous for the common 35delG mutation. There may also be progression with age. Mutation analysis in the GJB2 gene was performed on 36 families (group A) presenting with at least one individual with non-syndromic deafness (NSD). An unselected series of 26 deaf individuals referred by other services where the environmental factors were not completely excluded was also part of the study (group B). Mutations in the GJB2 gene were found in 22% (eight patients) of the families tested in group A, and 11.5% (three patients) of individuals within group B. This finding should facilitate diagnosis of congenital deafness and allow early treatment of the affected subjects.