Consumer interest in complementary and alternative medicine (CAM) has grown dramatically in Western countries in the past decade. However, very few patients volunteer information about CAM use unless directly questioned by their health-care practitioner. Therefore, by being informed about the prevalence and modality of CAM use for asthma, as well as characteristics of users, health-care practitioners may be better able to identify patients who use CAM. In turn, this may facilitate proactive discussion and optimization of the patient's overall asthma management. This review aims to summarize the current knowledge about use of CAM by people with asthma, and to assess the applicability of the available studies to the broader asthmatic population. Computerized literature searches were conducted on Medline, Embase, Cochrane and Allied and Complementary Medicine (AMED) databases from their inception to 13 April 2005. Search terms included: complementary medicine/therapies, alternative medicine/therapies and asthma. The bibliographies of accessible articles were searched for further papers. Seventeen studies have examined the use of CAM by people with asthma. The reported level of use for adults ranged from 4% to 79%, and for children from 33% to 89%. Among the most commonly used CAMs were: breathing techniques, herbal products, homeopathy and acupuncture. There is no strong evidence for effectiveness for any of these modalities. There is little consistency among available prevalence studies making conclusions difficult. Nevertheless, the high rates of CAM use reported in some studies indicate that CAM use should be taken into account when managing patients with asthma.