Ultraviolet A (UVA) irradiation modulates the immunological functions of skin. We examined the effect of UVA irradiation on the basal and the IFN-gamma-and TNF-alpha-stimulation-induced production of thymus-and activation-regulated chemokine (TARC/CCL17) using HaCaT cells. UVA irradiation inhibited the basal levels of both TARC mRNA expression and TARC protein production. UVA irradiation also significantly inhibited TARC mRNA expression and TARC protein secretion that were induced by co-stimulation with IFN-y and TNF-alpha. A time course study showed that: the significant suppression of TARC mRNA expression was detected 8 hours after irradiation and continued for 36 hours; the strongest inhibition of TARC protein secretion occurred in the first 8 hours after UVA irradiation and continued for 36 hours. Our data provide the first evidence that UVA inhibits TARC mRNA expression and TARC protein production by keratinocytes in a dose-dependent manner. These results may suggest an explanation for the UV-induced therapeutic effect.