We examined natural killer (NK) activity in 128 patients with atopic dermatitis (AD) to investigate the relationships between NK activity and severity of dermatitis, duration of disease, and mental states. The results showed the following: 1) No relationship was shown between severity of dermatitis and NK activity, neither between NK activity nor eosinophilic counts nor serum IgE. 2) Patients with longer duration of AD lesions showed significantly lower NK activity (P=0.036). The significant relationship was recognized between severity of dermatitis and the duration of disease (P=0.014). 3) No relationships were recognized between NK activity and mental states evaluated using the Profile Of Mood States (POMS) questionnaire, as tension-anxiety, depression-dejection, anger-hostility, vigor, fatigue and confusion. From a psychoneuroimmunological viewpoint, chronic stress as having AD might influence the lower NK activity of patients with longer duration of AD.