Patients with atopic dermatitis (AD) are known to suffer from nocturnal itch, and the resultant scratching may worsen the skin lesions. We observed nocturnal scratching for 112 nights in 35 adult patients with AD, using an infrared video camera system. To quantify the amount of scratching, we counted scratching bouts lasting more than 5 s and calculated the duration of all the scratching bouts (total scratching time, TST). The percentage of TST in the total recording time (TST%) was used as an index of nocturnal scratching. Mean +/- SD TST% was 14.3 +/- 13.9 for patients with severe AD, 6.2 +/- 3.7 for those with moderate AD and 0.7 +/- 0.4 for those with mild AD. The higher TST% in the severely affected group was attributed mainly to a longer duration rather than a higher frequency of bouts. Patients scratched more in the first third of the night than in the later two-thirds. Both the group of patients whose disease distribution pattern was generalized and those who showed a head-neck-shoulder type distribution scratched their heads, faces and necks for longer than other parts of the body. Repeated measurement performed on individual subjects resulted in a similar TST% when there was little change in skin lesions. TST% reduced by 15 +/- 21% when the patients showed marked improvement. The measurement of nocturnal scratching helps to evaluate the severity of itch in AD. In addition, the infrared video successfully detected the location and nature of nocturnal scratching in AD.