Decline in the prevalence of childhood deafness in the Jewish population of Jerusalem: ethnic and genetic aspects. A longitudinal study was performed on 147 Jewish children with bilaterally sensorineural hearing loss of moderately severe to profound degree, born in Jerusalem during the eighteen years 1968-85. The prevalence rate of these children declined during the years 1977-85, and at the same time the rate of consanguinity of their parents decreased; this decline was more evident in the genetic group among children with non-Ashkenazi ethnic origin. No such decline was found among the Ashkenazi children and no consanguinity among parents of these children was recorded. Our study supports the assumption that restriction of consanguineous matings may affect the prevalence of genetic deafness in children in a well-defined population. We have tried to remain unbiased and concede certain shortcomings in our present study. 