Omani society has a long tradition of consanguinity, which goes back to pre-Islamic times. The aim of the present study was to determine the prevalence of consanguineous marriage and the mean coefficient of inbreeding in Oman. Consanguinity rates among 60,635 couples (20.8% of the national population of childbearing age) were determined on the basis of a questionnaire in major delivery units. In this large survey, which included all sections of the community, 24.1% of marriages were reported between first cousins and 11.8% of marriages were between second cousins. In addition, a further 20.4% of marriages were contracted within specific tribal groupings. Because of the strictly endogamous nature of the tribal groups in Oman, all marriages would be expected to be consanguineous to some degree, albeit at a level beyond that of second cousins (F < or = 0.0156). To obtain a more detailed picture of the patterns of consanguinity, and to assess the validity of the questionnaire-based study, 500 pedigrees were investigated in detail. The mean coefficient of inbreeding (alpha) in these pedigrees was 0.0204, compared with 0.0198 in the larger survey, suggesting that the data were broadly comparable and confirming the high degree of consanguinity in the population of Oman.