We have examined the use of mitochondrial DNA (mtDNA) as a molecular marker to study the relation between chronologic aging and photoageing in human skin. Using a 3-primer quantitative polymerase chain reaction method we have studied changes in the ratio of the 4977 bp deleted to wild-type mtDNA in relation to sun exposure and chronologic age of human skin. Based on previous studies, samples showing greater than 1% deleted mtDNA were classed as abnormal. There was a significant increase in the incidence of high levels (i.e., >1%) of the 4977 bp deleted mtDNA in sun-exposed sites (27%, 27 of 100) compared with sun-protected sites (1.1%, one of 90) (Fisher's exact test, p < 0.0001). There appeared to be no relation between the frequency of the mtDNA deletion and age. Analysis of split skin samples showed that most deletions (93%, n = 27) were confined to the dermal rather than the epidermal component, and in keeping with this deletions were found in three of six primary cultures of fibroblasts from sun-exposed sites. Deletions were not seen in the epidermal component of several epidermal tumors nor were deletions seen in fibroblasts cultured from an individual with Werner's syndrome. We propose that deletions or mutations of mitochondrial DNA may be useful as a marker of cumulative ultraviolet radiation exposure.