Bronchial reactivity was studied via carbachol testing in 46 patients affected by type 1 diabetes mellitus and in 30 control subjects and via exercise testing in 55 diabetic patients and 60 control subjects. In all subjects, reactivity to skin allergen tests, the existence of atopic illnesses, and family history for these diseases were evaluated. Diabetic patients were less affected by asthma than control subjects, although their family history of atopy is greater. Bronchial reactivity, as expressed by the results of carbachol testing, was lower in diabetic patients than in control subjects. Furthermore, the duration of disease in diabetic patients was positively correlated with the results of carbachol testing. Reduced bronchial reactivity might be an indication of initial damage to the autonomic nervous system, which would thus appear to be both a complication and a protective factor.