Infused beta 2 agonists have been shown to cause focal myocardial necrosis. Serum creatine kinase activity was compared in 13 patients with brittle asthma currently being treated with subcutaneous terbutaline and an age and sex matched control group of patients with moderate asthma having inhaled treatment only. The median serum total creatine kinase activity for patients receiving subcutaneous terbutaline (211 units/l) was greater than that for the control group (120 units/l). The cardiac specific isoenzyme component of creatine kinase was not raised in either group, and the electrocardiograms and serum aspartate aminotransferase activity were normal. Electromyograms in five patients receiving subcutaneous terbutaline with high creatine kinase activity showed changes consistent with myositis in two, one of whom was subsequently shown to have a metabolic myopathy, which is thought to be long standing. No pathological changes were seen in the myocardium at necropsy in a patient who died from an acute attack of asthma while taking subcutaneous terbutaline. These results suggest that the raised creatine kinase activity seen in patients receiving this treatment is unlikely to be myocardial in origin.