Objectives: The aims of the study were to determine the prevalence, types and socio-economic correlates of consanguineous marriages in Iran, and to gauge the extent to which consanguinity influenced fertility, pregnancy outcomes and the expression of genetic disorders in the present-day population. Methods: Data on the prevalence of consanguinity and birth outcomes in the first marriages of 5,515 women were abstracted from the 2005 Iran Low Fertility Study [Hosseini-Chavoshi et al: Fertility and Contraceptive Use Dynamics in Iran: Special Focus on Low Fertility Regions. Canberra, Australian National University, 2007]. The results of associated socio-economic variables were collated and assessed by Pearson's ?2 analysis and logistic regression. Results: Overall, 37.4% of the marriages were consanguineous (? = 0.0149), but with major differences between 4 representative populations. Consanguinity was higher among rural couples, older marriage cohorts, women marrying at a younger age, and women with lower levels of formal education. In general, consanguineous couples had higher mean numbers of pregnancies, live births and surviving children. Conclusions: Given declining family sizes, a rapid urbanization and increased educational and employment opportunities, it seems inevitable that consanguineous marriages will decline in prevalence in Iran, albeit more slowly in more traditional rural communities. Predictably, there will be a concomitant reduction in the incidence of recessive genetic disorders, but this is against a background transition from communicable to non-communicable diseases.