Newborn male fraternal twins presented at 10 days of age will bilateral flank masses; intravenous urograms showed polycystic kidney disease. Both babies also had hypertrophic pyloric stenosis (HPS). Their father has radiographic and sonographi findings of previously unsuspected polycystic kidneys and has a history of HPS in infancy. The association of dominantly-inherited polycystic kidneys (DPK) and HPS in this family is probably due to chance. However the authors speculate that the autosomal gene for DPK may occur at one of several loci that carry the genetic liability for HPS, A DISORDER TRANSMITTED BY POLYGENIC INHERITANCE.