Recent experimental and epidemiological studies have suggested that outcomes of asthma are significantly influenced by treatment patterns. This study was conducted in order to investigate the links between treatment patterns in asthmatics and occurence of disease exacerbations. We performed a nested case-control study in a cohort of 680 asthmatics identified between 1986 and 1991 in a drug dispensing database. After validation in a pilot study, the intermittent use of oral corticosteroids was usedas a proxy for asthma exacerbations. Cases with an exacerbation (n=133) were pair-wise matched with controls. The type of medications used for the usual treatment of asthma were examined in relation to the risk of asthma exacerbation. The use of oral xanthines and inhaled fenoterol but not of inhaled salbutamol, corticosteroids, cromoglycate and ipratropium bromide was associated with an increased probability of asthma exacerbation. Within the cohort, the proportion of subjects dispensed inhaled corticosteroids rose from 12 to 27% between 1986 and 1991. The proportion of subjects using inhaled bronchodilators without inhaled corticosteroids also decreased over this period of time. The identification of markers of asthma exacerbations made it possibleto link the probability of adverse outcome risk for such exacerbations with treatment patterns. This method could be useful in further development of asthma surveillance using drug dispensing databases.