BACKGROUND Inhaled beclomethasone diproprionate (BDP) has been, together with inhaled budesonide, the mainstay of anti-inflammatory therapy for asthma for many years. A range of new prophylactic therapies for asthma is becoming available and BDP is now frequently used as the reference treatment against which these newer agents are being compared. OBJECTIVE The objectives of this review were to: a) Compare the efficacy of BDP with placebo in the treatment of chronic asthma. b) Explore the possibility that a dose response relationship exists for BDP in the treatment of chronic asthma. c) To provide the best estimate of the efficacy of BDP as a benchmark for evaluation of newer asthma therapies. METHODS We searched the Cochrane Airways Group Trial Register (1999) and reference lists of articles. We contacted trialists and Glaxo Wellcome for additional studies and searched abstracts of major respiratory society meetings (1997-1999). METHODS Randomised trials in children and adults comparing BDP to placebo in the treatment of chronic asthma. Two reviewers independently assessed articles for inclusion and methodological quality. METHODS One reviewer extracted data; authors were contacted to clarify missing information. Quantitative analyses where undertaken using Review Manager (Revman) 4.0.3 with Metaview 3.1. RESULTS 52 studies were selected for inclusion (3459 subjects). The studies were generally of high methodological quality. In non-oral steroid treated patients, BDP produced significant improvements in a number of efficacy measures compared to placebo including FEV1 weighted mean difference (WMD) 340ml (95% CI 190-500ml); FEV1 (% predicted) WMD 6% (95% CI 0.4 to 11.5%) and morning PEFR WMD 50 L/min (95% CI 8 to 92 L/min). BDP also led to reductions in rescue beta2 agonist use compared to placebo WMD 1.75 puffs/d (95% CI 1.4 to 2.4 puffs/d) and reduced the likelihood of trial withdrawal due to asthma exacerbation relative risk (RR) 0.26 (95% CI 0.15 to 0.43). In oral steroid treated patients BDP led to significantly greater reductions in oral prednisolone use WMD 5 mg/d (95% CI 4 to 6 mg/d) and a higher likelihood of discontinuing oral prednisolone RR 0.54 (95% CI 0.43 to 0.67). There was little evidence for a clincially worthwhile dose response effect, but few studies recruited patients with more severe asthma. CONCLUSIONS This review has quantified the efficacy of BDP in the treatment of chronic asthma and strongly supports its use. Current asthma guidelines recommend titration of dose to individual patient response, but the published data provide little support for dose titration above 400 mcg/d in patients with mild to moderate asthma. There are insufficient data to draw any conclusions concerning dose-response in patients with severe disease.