We evaluated the involvement of viral and Mycoplasma infections in severe attacks of asthma in 112 adult patients admitted to Oulu University Central Hospital for an exacerbation of asthma, during a period of one year. The total number of admissions was 151, and specimens for viral identification were collected from 142 of these. Although the methods for diagnosis of rhinoviruses and coronaviruses were lacking, the diagnosis of a viral or Mycoplasma infection was confirmed in 41 patients (29%) by one or more of the three methods; viral serology, viral culture from a throat wash or viral antigen detection from a nasopharyngeal aspirate. Thirty six of these patients (88%) with confirmed infection also had symptoms suggestive of a viral infection. We conclude that viral and Mycoplasma infections are so often involved in severe asthma attacks of adults that they may play an important role in the induction of such attacks.