The Authors hereby review the literature concerning hypertrophic pyloric stenosis, with particular reference to the genetic and familial influence. They present their cases of 49 children; out of these 3 pairs of biovular twins out of which only one of the twin babies was affected and 2 similar twins both of which were affected. In conclusion they are confirming the importance of a genetic component, influenced by environmental factors, on the origin of hypertrophic pyloric stenosis.