The Klippel–Trenaunay–Weber syndrome (KTWS) is generally thought to occur sporadically, following a somatic mutation model. However, in some cases, clinical manifestations of the syndrome have been found in family members, suggesting an autosomal dominant inheritance. Here we present an epidemiological analysis of a consecutive series of cases with KTWS identified in the Spanish Collaborative Study of Congenital Malformations (ECEMC). We found an increase in parental age and in the number of pregnancies, as well as familial occurrence of haemangiomas. These observations suggest a genetic contribution to the occurrence of KTWS.