Background Evidence suggests that brief physiotherapy programmes are as effective for acute whiplash-Associated disorders as more comprehensive programmes; however, whether this also holds true for chronic whiplash-Associated disorders is unknown. We aimed to estimate the effectiveness of a comprehensive exercise programme delivered by physiotherapists compared with advice in people with a chronic whiplash-Associated disorder. Methods PROMISE is a two group, pragmatic randomised controlled trial in patients with chronic (>3 months and <5 years) grade 1 or 2 whiplash-Associated disorder. Participants were randomly assigned by a computer-generated randomisation schedule to receive either the comprehensive exercise programme (20 sessions) or advice (one session and telephone support). Sealed opaque envelopes were used to conceal allocation. The primary outcome was pain intensity measured on a 0-10 scale. Outcomes were measured at baseline, 14 weeks, 6 months, and 12 months by a masked assessor. Analysis was by intention to treat, and treatment effects were calculated with linear mixed models. The trial is registered with the Australian New Zealand Clinical Trials Registry, number ACTRN12609000825257. Findings 172 participants were allocated to either the comprehensive exercise programme (n=86) or advice group (n=86); 157 (91%) were followed up at 14 weeks, 145 (84%) at 6 months, and 150 (87%) at 12 months. A comprehensive exercise programme was not more effective than advice alone for pain reduction in the participants. At 14 weeks the treatment effect on a 0-10 pain scale was 0·0 (95% CI -0·7 to 0·7), at 6 months 0·2 (-0·5 to 1·0), and at 12 months -0·1 (-0·8 to 0·6). CNS hyperexcitability and symptoms of post-traumatic stress did not modify the effect of treatment. We recorded no serious adverse events. Interpretation We have shown that simple advice is equally as effective as a more intense and comprehensive physiotherapy exercise programme. The need to identify effective and affordable strategies to prevent and treat acute through to chronic whiplash associated disorders is an important health priority. Future avenues of research might include improving understanding of the mechanisms responsible for persistent pain and disability, investigating the effectiveness and timing of drugs, and study of content and delivery of education and advice.