Genome duplication inevitably results in replication errors. A priori, the more times a genome is copied, the greater the average number of replication errors. This principle could be used to 'count' mitotic divisions. Although somatic mutations are rare, cytosine methylation is also copied after DNA replication, but measurably increases with aging at certain CpG rich sequences in mitotic tissues, such as the colon. To further test whether such age-related methylation represents replication errors, these CpG rich 'clock' sequences were measured in leucocytes. Leucocytes within an individual have identical chronological ages (time since birth) but their mitotic ages (numbers of divisions since the zygote) may differ. Neutrophils, B-lymphocytes, and red cell progenitors are produced from relatively quiescent stem cells throughout life, but T-lymphocyte production largely ceases after puberty when the thymus disappears. However, T-lymphocyte genomes may continue to replicate throughout life in response to immunological stimulation. Consistent with this biology, clock methylation significantly increased with aging for T-lymphocyte genomes, but no significant increase was measured in other cell populations. Moreover, this methylation was greater in genomes isolated from their corresponding neoplastic populations. These studies tentatively support the hypothesis that methylation at certain CpG rich sequences in leucocytes could record their mitotic ages.