The inference of human genetic ancestry based on mitochondrial DNA (mtDNA) haplogroups has been known to be less reliable than that based on genome-wide single-nucleotide polymorphism (SNP) platforms, but the extent of this has not been well characterized. Emery et al. now provide a quantification of this in analyses of individuals with the same mtDNA haplogroup from worldwide populations, making use of data sets from the Human Genome Diversity Project and the 1000 Genomes Project. They find that although mtDNA haplogroup membership alone provides some ancestry information, it has limited ability to predict continental ancestry and geographical origin. The authors recommend that these limitations should be made clear to consumers of commercial lineage-based ancestry testing products.