Background: It is knovn that Langerhans cells are damaged fuctionally and morphologically by UV irradiation. Recently, high-dose UVA-1 therapy (340-400nm) was introduced as an effective treatment of severe exacerbated atopic dermatitis. However, the effect of UVA-1 therapy on surface markers and function of epidermal Langerhans cells are still unclear. Objective : To determine whether a high dose UVA-1 irradiation affects cutaneous immune system, the effect of UVA-1 on the expression of ATPase and Ia antigen of mouse epidermal Langerhans cells and induction of contact hypersensitivity in mice skin were investigated and were compared to those of UVA-2. Methods : Balb/c mice were irradiated with 150J/cm and 300J/cm of UVA-1 and UVA-2 in a single dose at one time or 3 fractionated doses for 3 days. The number of Langerhans cells was evaluated using ATPase and immunoperoxidase-stained epidermal sheets. Balb/c mice were irradiated with same manner after induction of contact hypersensiyity by applying 0.5% oxazolone solution and the influence of UV irradiation was evaluated by measuring the ear swelling of mice. Results : 1. The expression of surface markers of Langerhans cells was not affected by 150J/cm and fractionated 300J/cm of UVA-1. However, single irradiation of 300J/cm of UVA-1 reduced signifi-cantly the expression of surface markers. The irradiation of UVA-2 induced more prominent reduction of the expression of surface markers compared to UVA-l. 2. Although the induction of contact hypersensitity was not inhibited in groups irradiated by single or fractionated 150J/cm of UVA-1, it was inhibited in groups irradiated with 300J/cm of UVA-1. The inhibition of contact hypersensitivity induction by UVA-2 irradiation was also more prominent than that by UVA-1. Conclusion : These results suggest that epidermal Langerhans cells could be damaged by high doses of UVA-1 and the damage of Langerhans cells by UVA-1 is weaker than that by UVA-2. (Kor J Dermatol 1996;34(4): 637-644)