Equivalence trials aim to show that two treatments have equivalent therapeutic effects. The approach is to define, in advance, a range of equivalence -d to +d for the treatment difference such that any value in the range is clinically unimportant. If the confidence interval for the difference, calculated after the trial, lies entirely within the interval, then equivalence is claimed. Glaxo Wellcome has carried out a series of trials using this methodology to assess new formulations of inhaled beta-agonists and inhaled steroids in asthma. Eleven of these trials are used to review some practical issues in equivalence trials. For the series of asthma trials, a range for peak expiratory flow rate (PEF) from -15 to +15 l/min was chosen to be the range of equivalence. This fitted well with physicians' opinions and with previously demonstrated differences between active and placebo. The choice of the size of the confidence interval should depend on the medical severity of the clinical endpoints under consideration and the level of risk acceptable in assuming equivalence if a difference of potential importance exists. From this point of view, a recommendation in the CPMP Note for Guidance on Biostatistics that 95 per cent confidence intervals should be used is inappropriate. Intent-to-treat (ITT) and per-protocol (PP) analyses were compared for the eleven asthma trials. Confidence intervals were always wider for the PP analysis and this was entirely due to the smaller number of subjects included in the PP analysis. There was no evidence that the ITT analyses were more conservative in their estimates of treatment difference. The need to demonstrate equivalence in both an ITT and a PP analysis in a regulatory trial increases the regulatory burden on drug developers. The relative importance of the two analyses will depend on the definitions used in particular therapeutic areas. Demonstrating equivalence in one population with strong support from the other would be preferred from the Industry viewpoint. In trials with regulatory importance, prior agreement with regulators on the role of ITT and PP populations should be sought. Trial designs will need to take account of the estimated size of the PP population if adequate power is needed for both analyses. Careful design in the series of asthma trials, particularly identifying a population of patients with potential to improve, resulted in notable increases in lung function during the course of the trials for both treatments. This provided reassurance that equivalence was not due to a lack of efficacy for both treatments. In one trial equivalence was demonstrated overall but a treatment by country interaction was noted. However, this interaction could not be attributed to differences in patient characteristics or baseline data between the countries. Study conduct was also similar in the different countries. The conclusion was that the interaction was spurious and that the trial provided good evidence of equivalence.