We previously reported a simple and easy-to-use device, Scratch Monitor for evaluation of nocturnal scratching. In the present study, the effect of an antihistamine (azelastine hydrochloride) on atopic dermatitis was investigated using this device. In 40 patients with atopic dermatitis, nocturnal scratching was measured by the monitor, when taking the drug and while off medication. Neither the 'sleeping time' nor the 'pre-asleep time' differed significantly, but there was a significant difference in 'scratch rate', 'minute scratch records', 'hourly awake number' and 'awake rate', which indicated that this drug alleviated nocturnal scratching and sleep disturbance. The difference was most prominent especially in patients with a serum IgE > or = 1000 IU/ml as well as in patients with mild symptoms and normal serum lactate dehydrogenase levels. The drug was more effective for nocturnal scratching in the early period, when the 'sleeping time' was divided into three equal parts (early, mid and late periods) and each period was compared.