Atopic Dermatitis (AD) and asthma are closely associated with respect to epidemiology, hereditary factors and occurrence in the same individuals. Bronchial Hyperresponsiveness (BH), the hallmark of asthma, can also be a physiopathological feature of AD, even in the absence of clinical asthma. We studied 78 subjects with AD. A follow-up study was performed in 27 of these. Data on respiratory and dermatologic symptoms were collected by means of a standardized questionnaire. Skin reactivity was evaluated by prick testing, and in 57 subjects BH was assessed with a methacholine test (Mch). Twenty-one subjects had asthma and 36 showed a positive skin reaction. A PC20 FEV1 was measurable in 38 subjects. Males were found more likely to be Mch responders than females (p < 0.05). Mch responders also showed an earlier age at onset of AD than nonresponders (2.1 yrs vs. 6.2, p = 0.03). Determinants of the degree of BH were evaluated by a stepwise multiple regression analysis, taking the log of the slope of the concentration response curve as dependent variable. In the final model we found that the degree of BH was directly related to wheezing (p = 0.0017) and coughing (p = 0.04) and inversely related to lung function (p = 0.0082) and age (p = 0.0008). Neither skin reactivity nor grading of AD were statistically significant. The longitudinal study demonstrated that the courses of AD and BH seem to run parallel only in skin-negative subjects, whereas an increase in BH was observed in skin-positive subjects.