Obesity and asthma are common chronic diseases in the industrialised world. The aim of the present study was to investigate a possible relationship between asthma and obesity among persons on sick leave due to respiratory disorder. The diagnosis of asthma (n = 237) was made in a three-stage model (questionnaire, telephone interview and clinical examination) in persons on sick leave due to any respiratory disorder. Persons on sick leave due to non-specific spinal pain, (n = 1231) and a general population sample (n = 5092) were used as references. Obesity (body mass index > 30 kg/m2) was found in 20.7% of the asthmatic patients on sick leave compared with 13.7% in the non-specific pain patients on sick leave and in 6.5% of the controls (P < 0.001). It is not clear whether the increased prevalence of obesity among asthmatics reflects a true increase in asthma in obese persons or whether asthma-like symptoms occur because of obesity. Weight reduction schemes and weight maintenance programmes should be important components in treatment and rehabilitation plans for persons with asthma.