Data were obtained on surnames of the parents and places of birth of the parents and grandparents of children in Siena, Italy. Isonymy and total inbreeding coefficient, and their random and non-random components, are 0·005, 0·00125, 0·00019 and 0·00106, respectively. Isonymy and inbreeding figures are similar to those of other medium-sized Italian towns, while higher values have been reported for Italian villages and Italian ethnic minorities. City endogamy, and endogamy of Contrada for grandparents have the same values (44·1 and 44·8%, respectively), but for parents, endogamy of Contrada is lower than city endogamy (15·2 and 33·4%, respectively). The difference between the extent of Contrada endogamy expected at random and observed in the parents' generation does not seem to affect the genetic structure of the present population. However, the bulk of marriage migration (more than 70%) is short range, with people coming from Tuscany. There is no statistical difference in marital migration between males and females.