Defensins and cathelicidins (LL-37) are major antimicrobial peptides (AMPs) of the innate immune system of the human skin. In normal non-inflamed skin these peptides are negligible, but their expression can be markedly increased in inflammatory skin disease such as psoriasis. We designed this study to identify the expressions of LL-37 in normal human keratinocyte (NHK) and HaCaT cells after exposure to stimulants and to investigate difference of LL-37 expression accompanied with cell differentiation status, and come to understand difference of susceptibility to infection in atopic dermatitis and psoriasis. Expressions of LL-37 in NHKs and HaCaT cells were evaluated by using RT-PCR, Western blotting, and immunohistochemical (IHC) staining at 6, 12, and 24 hr post stimulation after exposure to Ultraviolet B irradiation and lipopolysaccharide. And expression of LL-37 in skin biopsy specimens from patients with atopic dermatitis and psoriasis was determined by immunohistochemical analysis. In time-sequential analyses of LL-37 expression revealed that LL-37 was expressed in NHKs, but not in HaCaT cells. IHC analysis confirmed the presence of abundant LL-37 in the epidermis of psoriasis. Therefore we deduced that expression of LL-37 is affected by UV irradiation, bacterial infection, and status of cell differentiation.