In 32 mainly atopic children (aged 9-16 years) staying in a mountain resort the effects of viral infections on the course of asthma were studied. Daily surveillance was made of respiratory infection symptoms and clinical parameters for asthma, including forced expiratory volume in 1s (FEV1). Serology for several respiratory viruses. Mycoplasma pneumoniae and Chlamydia psittaci was done. In 58 symptomatic respiratory infections (SRI) 39 asthmatic exacerbations occurred (67%). Eighteen SRIs of viral origin caused 16 exacerbations (89%). Twenty-one seroconversions with mild or absent symptoms of infection gave six exacerbations (29%). Five flare-ups of asthma in eight episodes of seroconversion (63%) during which patients were not under observation were reported. Streptococcal pharyngitis did not cause asthmatic reactions. Thirteen SRIs due to influenza A all caused asthmatic exacerbations. The similarity of these data to those in children with "intrinsic" asthma suggests that the development of viral excerbations in asthmatics is not determined by whether the patient is allergic or not.