The north-western populations of Pakistan in the Federally Administered Tribal Areas (FATA) adjoining the Pakistan–Afghanistan border are an amalgamation of native and migrated Pashtun tribes. These tribal populations are in transition due to war conditions and geo-political turmoil on both sides of the border since the Soviet invasion in 1979. Bio-demographic and epidemiological data for these tribes are scarce. A prospective cross-sectional sample of 967 males was selected from a representative Pashtun population of Bajaur Agency, and information obtained on bio-demographic variables and marital union types. Analysis of these data revealed that consanguinity was 22.34% and the inbreeding coefficient F was calculated to be 0.0134. The inbreeding coefficient was observed to be higher in subjects who were illiterate, had unskilled jobs and who belonged to younger age categories, extended families and the Tarkalani tribe. Further analyses with respect to temporal variables like subject's age, year of marriage and age at marriage revealed that after a transition in marital union types in the early 80s, there has been a declining trend in the rate of consanguineous unions. Further, consanguineous unions in the parental generation were only 5%, but parental marriage types were predictors of subjects' marital union types. The data further establish that, contrary to a general notion about a high consanguinity rate in Pakistan, consanguineous unions are not common in Bajaur Agency and first cousin marriage is not the preferred type. Furthermore, this research shows that there is a great regional variation in the pattern of consanguinity in Pakistan that needs to be documented in order to draw a more comprehensive picture of the inbreeding coefficient in the country.