Recombinant human DNase has been shown to reduce sputum viscosity and improve pulmonary function in patients with cystic fibrosis.1 The individual response is, however, highly variable with between one third and two thirds of patients showing an improvement in their forced expiratory volume in one second (FEV₁) of more than 10%.1 In view of the high cost of this treatment and the lack of long term studies confirming its safety, prediction of the response of individual patients is an important issue. In a larger series of patients both baseline pulmonary function and the clinical …