Patients with chronic conditions are increasingly using complementary therapies. Asthma is the most common chronic disease in the UK. Qualitative research has suggested reasons why asthma patients use complementary therapies. However, there is little reliable quantitative evidence regarding the prevalence of complementary therapy use among asthma patients and predictors of use. A postal survey of complementary therapy use among asthma patients was therefore conducted via 27 general practices across seven Primary Care Trusts within the South West Strategic Health Authority (England), during August 2005 to May 2006. A total of 14 833 asthma patients were identified. A 1-in-4 random sample generated 3693 potential respondents, of whom 1320 (36%) returned questionnaires. Taking full account of the survey design, 14.5% (190/1308; 95% confidence interval 12.5% to 16.6%) had used complementary therapies for asthma; 54% of these patients had not disclosed their complementary therapy use to a health professional. The three therapies most commonly used were homeopathy, herbal medicine and relaxation. Just over half of those using complementary therapies for asthma reported that they usually or always helped; the most common reported benefits were symptom reduction, calming breathing and reducing panic. Multivariable analyses indicated an inverted U-shaped relationship between complementary therapy use for asthma and age, and increased likelihood of use among women, those with educational qualifications, those not usually helped by asthma medication, and those who have difficulty sleeping because of asthma symptoms. Dissatisfaction with conventional care was not associated with complementary therapy use for asthma. Asthma patients may use complementary therapies with or without the knowledge of their healthcare providers. Open communication between professionals and patients about complementary therapies may be valuable to give patients enhanced opportunities to discuss the impact of asthma on their quality of life and the effectiveness of their conventional treatment.