An outbreak of acute asthma occurred in Birmingham and the surrounding area on July 6 and 7, 1983. In most patients symptoms began at the time of sudden climatic changes associated with a thunderstorm. Air pollution was not a factor. The large and sudden increase in numbers of airborne fungal spores, especially Didymella exitialis and Sporobolomyces, around the time of the outbreak suggests that they may have been partly contributory, although a direct causal effect has not yet been established.