Somatic mitochondrial DNA (mtDNA) mutations accumulate with age in postmitotic tissues but have been postulated to be diluted and lost in continually proliferating tissues such as bone marrow (BM). Having observed marked sequence variation among healthy adult individuals' total BM cell mtDNA, we undertook analysis of the mtDNA control region in a total of 611 individual CD34+ clones from 6 adult BM donors and comparison of these results with the sequences from 580 CD34+ clones from 5 umbilical cord blood (CB) samples. On average, 25% (range, 11% to 50%) of individual CD34+ clones from adult BM showed mtDNA heterogeneity, or sequence differences from the aggregate mtDNA sequence of total BM cells of the same individual. In contrast, only 1.6% of single CD34+ clones from CB showed mtDNA sequence variation from the aggregate pattern. Thus, age-dependent accumulation of mtDNA mutations appears relatively common in a mitotically active human tissue and may provide a method to approximate the mutation rate in mammalian cells, to assess the contribution of reactive oxygen species to genomic instability, and for natural "marking" of hematopoietic stem cells; our data also have important implications for the aging process, forensic identifications, and anthropologic conclusions dependent on the mtDNA sequence.