Epidemiological data for the eighth asthma epidemic in Barcelona were available from the whole city. They reveal a striking space clustering, an hour-time clustering, and simultaneous time and space clustering, which indicate that the outbreak was a point-source epidemic. Air pollution by smoke and oxides of sulphur and nitrogen was unlikely to have been the cause of the epidemic, as was an unusual level of pollen or fungal spores. The industrial area adjacent to the neighbourhood in which the epidemic occurred was a probable source of the cause of the outbreak.