Nebulized recombinant human deoxyribonuclease (DNase) reduces sputum viscosity and improves lung function in some cystic fibrosis patients, but individual responses are unpredictable. The aim of this study was to investigate how DNase can be targeted to those cystic fibrosis patients who would benefit most. The Scottish Cystic Fibrosis Group agreed on a randomized, double-blind, placebo-controlled n-of-1 assessment protocol. Patients underwent a maximum of three 4-week assessment periods (2 weeks saline, 2 weeks DNase each). Measurements performed at hospital (exercise, oximetry and spirometry) and home (symptom scores) were used to derive a scoring system to discriminate maximally between DNase and placebo effects. The data on 89 4-week assessments in 52 patients were reported. Twenty-four patients have completed the assessment process (12 responders and 12 nonresponders) to date. Forced expiratory volume in one second (FEVI) was the best discriminator of response, rising by >200 mL after DNase in 33 of 89 (37%) assessments compared with 3 of 89 (3%) after saline. N-of-1 trials, while laborious, permitted genuine treatment effects to be quantified within individuals with confidence, permitting appropriate treatment targeting. This provides a model of how other new expensive therapies may be introduced to maximize patient benefit.