Some patients with atopic dermatitis (AD) develop dry skin or exacerbated cutaneous inflammations with frequent swimming in public pools or after bathing. We examined the effects of residual chlorine in bathing water on the function of the stratum corneum (SC) in patients with AD and determined the lowest chlorine concentration showing an effect. In addition, we investigated the relationship between the free residual chlorine concentration in bathing water and the water-holding capacity of the SC in patients with AD. Twenty patients with AD and 10 normal control (NC) subjects were included in this study. The hydration status of the SC on the flexor surface of the forearm was measured with a corneometer before and after the subject's arms were immersed in tubs filled with comfortably hot water (40 degrees C) containing residual chlorine at concentrations of 0, 0.5, 1.0 and 2.0 mg/L for 10 minutes in a room maintained at normal temperature (24 degrees C) and relative humidity (55%). The water-holding capacity of the SC after immersion was calculated by integration of the hydration status determined every 30 seconds over a period of 10 minutes. In the patients with AD, the average SC hydration status after immersion in comfortably hot water containing residual chlorine at 1.0 and 2.0 mg/L was significantly lower than that following immersion in water containing a negligible concentration of residual chlorine (i.e., less than 0.03 mg/L) (p<0.05). In the NC subjects, significant differences were observed only between the 2.0 mg/L and the negligible residual chlorine groups (p<0.05). The water-holding capacity of the SC was significantly decreased with a residual chlorine concentration of 0.5 mg/L or higher in the patients with AD (p<0.01). However, in the NC subjects, a significant decrease in water-holding capacity was observed only at a residual chlorine concentration of 2 mg/L (p<0.01). These results indicate, first, that the water-holding capacity of the SC in patients with AD is more sensitive to free residual chlorine exposure than that in NC subjects without AD. Second, these results suggest that free residual chlorine exposure in patients with AD may play a role in the development or exacerbation of AD.