During the 1987 Australian winter, respiratory illness patterns were studied in a population of 454 healthy adults, aged 18-59, over a period of 45 days. These patterns were matched with data obtained from laboratory diagnoses for respiratory viruses, Mycoplasma pneumoniae and bacteria. Influenza B/1/86 was by far the most prevalent pathogen but other viruses including influenza A, paramyxoviruses, respiratory syncytial virus and coronavirus OC-43 were also present, either alone or in combination during the sampling period. Overall, 92 males and 101 females experienced one episode, 12 males and 22 females experienced two episodes and four females experienced three episodes. However, there were only 52 instances of viral or M. pneumoniae infections, of which 37 had a defined aetiology, while the remainder were clinically silent. No bacterial pathogens could be detected from throat swabs taken from 15 of 37 volunteers in whom a viral infection was detected, or from 43 of 70 volunteers who did not experience such infections. The study indicates that major deficiencies in our understanding of the aetiology of respiratory viral illness are probably due to methodological problems in obtaining laboratory diagnoses for many respiratory viruses, and that great difficulties exist in establishing an aetiology for respiratory infections based upon clinical symptoms alone.