Ethnicity has been associated with the incidence of diseases and consequently it is a cornerstone in medical genetic studies. It is mainly important in admixture populations, where the population stratification can produce spurious results that lead to erroneous conclusions. Consequently, population stratification has become one of the most important confounding factors in populationbased genetic association studies, especially in Latino populations. Cartagena de Indias is a cosmopolitan city with dissimilar ancestry proportions due to recent miscegenation. This population mainly exhibits African and Amerindian matrilineal ancestries. Nevertheless, important asymmetries in the paternal genetic history related to the complex patterns of migration in the colonial period increase the male genetic diversity in this population. As a result of this recent admixture, population stratification has arisen, where each subpopulation is not equally represented. Consequently, the allele differences between cases and controls could be related with different frequencies among different population strata rather than the association of the genes with the disease. In order to define the patrilineal substructure of the Cartagena's population, a total of 130 unrelated men were ancestrally studied using 15 Y-STR loci routinely employed in anthropological, forensic and population genetics. Our results show that Cartagena is an admixture population consisting of European (80%), Amerindian (10%) and African ancestries (10%), which are