Congenital deafness affects 0.05-0.1% children. 90% of them were born from parents with normal hearing. There were no hearing defects in their families. In 70% of deafness cases deafness is the only symptom of the disease and is thought to be non-syndromal. Hypoacusis of unclear etiology may be congenital in 50% cases. Half of cases of severe autosomic-recessive non-syndromal hypoacusis (deafness) appear because of changes in only one gene--gene of connexine-26. Two thirds of the defects in this gene arise because of one mutation--35delG. Mean rate of this mutation carriage in Russia is over 2%. Identification of mutation 35delG in gene Cx26 is performed with the use of polymerase chain reaction. Genetic examination of 75 children with isolated hypoacusis (deafness) has detected deletion 35delG in both gene copies in 23 children (30%). 10 patients (13.4%) had mutation 35delG only in one copy of gene Cx26. Hearing defects in the latter may be related with the presence of another mutation in the same gene. A total of 33 patients (42%) carried deletion 35delG. According to our findings, the changed genotype is characterized primarily by bilateral neurosensory hypoacusis of the third-fourth degree. Weaker loss of hearing is rare. Thus, mutations in connexine gene26 present a problem for parents with normal hearing. Therefore, families with a deaf child should be referred for medicogenetic consultations.