This study was designed to assess the clinical therapeutic effect of persimmon [Diospyros kaki Thunberg (Ebananceae)] leaf extract and astragalin in NC/Nga mice suffering from dermatitis and the dose–response preventive effects of persimmon leaf extract on dermatitis and trans-epidermal water loss (TEWL). The efficacy of persimmon leaf extract or astragalin in NC/Nga mice was judged by measurement of skin severity, scratching behaviour, serum IgE levels or TEWL. Oral administration of persimmon leaf extract (250 mg/kg) or astragalin (1.5 mg/kg) for 4 weeks into NC/Nga mice with overt dermatitis resulted in a decrease in the severity of the condition. The preventive effect of persimmon leaf extract on the dermatitis was dose-dependent. In addition, TEWL was also suppressed at a persimmon leaf extract dose of 250 mg/kg.