Abstract:  NC/Nga mice have pathological and behavioral features similar to those seen in human atopic dermatitis. There are two known dermatitis models in NC/Nga mice, one being spontaneous‐induced dermatitis under conventional conditions and the other 2,4,6‐trinitrochlorobenzene (TNCB)‐induced allergic contact dermatitis. However, there are significant differences in time course on development of dermatitis. We studied the role of scratching behavior (sign of itch) on the development of dermatitis on spontaneous‐ and TNCB‐induced dermatitis. We measured scratching counts, transepidermal water loss (TEWL), and skin inflammation score, under conventional conditions or by applying 5% TNCB once a week for 6 weeks in NC/Nga mice. In spontaneous‐induced dermatitis, scratching counts increased with the passage of time. The scratching counts were significantly increased only 1 week after housing the mice under conventional conditions, but no changes were observed in cases of TNCB‐induced dermatitis. In spontaneous‐induced dermatitis, TEWL and skin‐inflammation score were gradually increased, time‐dependently. On the other hand, in TNCB‐induced dermatitis, these dependent values rapidly increased and reached a maximum only after 24 h TNCB application. These data suggest that pathogenesis of spontaneous‐ and allergic contact‐induced dermatitis was clearly different. It will be of major interest to identify the pruritic mediators causing profound scratching behavior and scratching‐induced aggravation of inflammation in the spontaneous‐induced dermatitis, as opposed to the inflammatory mediators that cause contact allergic dermatitis without major scratching.