We present two cases of dizygotic male twins and two cases of male first cousins with infantile hypertrophic pyloric stenosis (IHPS). All patients were treated with open pyloromyotomy. No complications were reported. The patients had the same risk factors for IHPS. First, all patients were first-born white males. Second, the twins were preterm (35 weeks). Third, the twins and the first cousins were exclusively bottle feeding. Thus, a combination of genetic and environmental factors may have contributed to the appearance of IHPS.