The spectrum of postmortem damage in mitochondrial DNA was analyzed in a large data set of cloned sequences from ancient human specimens. The most common forms of damage observed are two complementary groups of transitions, termed "type 1" (adenine-->guanine/thymine-->cytosine) and "type 2" (cytosine-->thymine/guanine-->adenine). Single-primer extension PCR and enzymatic digestion with uracil-N-glycosylase confirm that each of these groups of transitions result from a single event, the deamination of adenine to hypoxanthine, and cytosine to uracil, respectively. The predominant form of transition-manifested damage varies by sample, though a marked bias toward type 2 is observed with increasing amounts of damage. The two transition types can be used to identify the original strand, light (L) or heavy (H), on which the initial damage event occurred, and this can increase the number of detected jumping-PCR artifacts by up to 80%. No bias toward H-strand-specific damage events is noted within the hypervariable 1 region of human mitochondria, suggesting the rapid postmortem degradation of the secondary displacement (D-loop) H strand. The data also indicate that, as damage increases within a sample, fewer H strands retain the ability to act as templates for enzymatic amplification. Last, a significant correlation between archaeological site and sample-specific level of DNA damage was detected.