Obstructive bronchitis of young babies and infants is most often caused by respiratory-syncytial (RS)- and parainfluenza viruses, to a certain extent by adenoviruses. In the school age-group rhinoviruses and to a smaller extent mycoplasma pneumoniae gain further importance. All the viruses may cause or deteriorate asthmatic attacks, as well as during common cold or on the basis of documented allergies. Proposed mechanisms of viral-infection-induced obstructive airway disease are distinct lesions of bronchial-epithelium, followed by enhanced allergen-resorption and sensitization of submucosal mast cells, establishing an allergy. Following epithelial discontinuation airway irritant receptors which are raised in sensitivity by viral toxins are exposed and stimulated. Via vago-vagal reflexes this leads to bronchoconstriction. Furthermore, in the development of bronchial hyperresponsiveness there may be participation of virus-induced blockage of adrenoceptors, basophilic and mast-cell release of mediators as well as induction of specific IgE antibodies against RS- and/or Para-influenza-viruses. Viral airway infections are supposed to influence the immunological response of T-lymphocytes and probably macrophages selectively, e.g. to regulate IgE-antibody production. Especially in children with an atopic family history the RS-viruses cause obstructive airway disease. During acute respiratory-virus infections using electron microscopy marked pathological changes of bronchial-epithelium can be demonstrated causing disturbance or even stopage of mucus-transport. These changes are fully reversible within weeks after airway infection. Although there is no doubt about a causative relation between respiratory-tract infection and allergy, hardly any prediction for development of asthma bronchiale in later life, after having suffered from virus-induced obstructive airway disease early in life, can be made.(ABSTRACT TRUNCATED AT 250 WORDS)