We have reported that xeroderma pigmentosum group A (Xpa) gene-knockout mice [Xpa (-/-) mice] are deficient in nucleotide excision repair (NER) and highly sensitive to UV-induced skin carcinogenesis. Although xeroderma pigmentosum group A patients show growth retardation, immature sexual development, and neurological abnormalities as well as a high incidence of UV-induced skin tumors, Xpa (-/-) mice were physiologically and behaviorally normal. In the present study, we kept Xpa (-/-) mice for 2 years under specific pathogen-free (SPF) conditions and found that the testis diminished in an age-dependent manner, and degenerating seminiferous tubules and no spermatozoa were detected in the 24-month-old Xpa (-/-) mice. In addition, a higher incidence of spontaneous tumorigenesis was observed in the 24-month-old Xpa (-/-) mice compared to Xpa (+/+) controls. Xpa (-/-) mice provide a useful model for investigating the aging and internal tumor formation in XPA patients.