Confounding due to population stratification is a potential source of concern in population-based genetic association studies, particularly in recently admixed populations such as African Americans. Several methods have been developed to control for population stratification in the context of genome-wide association studies. Because these approaches require thousands of genotypes from genetic markers, they are not well suited to be used in genetic association analyses without genome-wide data. An alternative approach to control for population stratification is to estimate admixture proportions by using ancestral informative markers (AIMs). The authors evaluated whether a relatively small number of AIMs would be sufficient to estimate ancestral proportions in African Americans. They first estimated European admixture proportions in 1,757 subjects from the Black Women's Health Study (1995-2009) by genotyping an admixture panel of 1,373 AIMs; they then compared these results with those obtained using smaller sets of AIMs. The authors found that just 30 AIMs are needed to obtain very high correlation of estimates with the entire set (r = 0.89; P < 0.0001). A set of 200 AIMs gave an almost perfect correlation with the entire set (r = 0.98; P < 0.0001). These results show that a small number of AIMs are sufficiently precise to estimate European admixture in African Americans.