Asthma is a chronic disease estimated to affect 6-7% of the total UK population. In addition, a number of studies have shown that asthma has become commoner since the 1970s, especially in children. The diagnosis of asthma can be difficult and its management requires the involvement of patients in a long-term treatment plan, something which general practitioners may be unable to achieve easily in the average 10-min consultation. As a consequence, asthma is underdiagnosed and undertreated. Deaths from the disease are often avoidable with timely and sufficient use of the available medication. In order to support this, the British Thoracic Society (BTS) has published guidelines for asthma management based upon a stepwise approach, in which a patient is categorized as being on one of five steps according to the severity of his or her asthma. The guidelines give "rules of thumb" for deciding when the patient should move up or down the steps. The most recent version of the guidelines also included special rules for children. Within a recent European Community project on Advanced Informatics in Medicine (AIM), we developed a prototype decision support system for asthma management targeted at the primary care setting and based on the British Thoracic Society guidelines. This paper reports this development, and describes the further work needed on the prototype. Plans for evaluation of the knowledge bases and for future full application production are also described.