Mutations of mitochondrial (mt) DNA such as the 4977 base-pair large-scale deletion, also called common deletion, are increased in photoaged skin. Direct evidence for their induction by chronic exposure to ultraviolet (UV) radiation in vivo in human skin has remained elusive however. Furthermore, their fate after induction is unclear. Previously unirradiated skin of 52 normal human individuals was repetitively exposed to physiological doses of UVA light. Skin and blood specimens were investigated for the presence of mtDNA mutations employing semiquantitative nested PCR, as well as real-time PCR, after 2 weeks of UV exposure and the content of the common deletion was followed up for up to 16 mo after cessation of irradiation. As assessed by both methods, repetitive UV exposure led to an approximately 40% increase in the levels of the common deletion in normal human skin. The majority of deletions were detectable in the dermis also showing the biggest increase, whereas in the epidermis only residual levels and no increase were found. Nine individuals were examined up to 16 mo after cessation of UV exposure and some showed accumulation up to 32-fold. Thus, mtDNA mutations are induced in the human skin by repetitive UV exposure. In addition, these mutations seem to represent long-term in-vivo biomarkers for actinic damage in the human skin.