A reduction in the post-ultraviolet (UV) DNA repair capacity is associated with aging. To clarify the mechanism of this change, we examined the DNA repair capacity of skin fibroblasts from healthy donors of different ages by the two methods: host cell reactivation (HCR) assay and ELISA of cyclobutane pyrimidine dimers and pyrimidine-pyrimidone (6-4) photoproducts. In HCR assay, cells from elderly donors exhibited significant declines in the ability to restore transfected reporter DNA damaged by UV light. In contrast, the ability to remove DNA damage declined little with age in ELISA. These results imply that the age-sensitive step took place after the damage excision in nucleotide excision repair (NER). The mRNA expression of DNA repair synthesis-related genes (DNA polymerase delta, replication factor C, and proliferating cell nuclear antigen) were markedly decreased in the cells from multiple elderly subjects compared with those from young subjects. Further, the protein level of DNA polymerase delta1, a catalytic subunit of the pivotal factor in repair synthesis, correlated with the mRNA level. These findings suggest that the reduced post-UV DNA repair capacity in aging results from an impairment in the latter step of NER by the decreased expression of factors in repair synthesis.