Preference for a particular intervention may, possibly via complicated pathways, itself confer an outcome advantage which will be subsumed in unblind randomised trials as part of the measured effectiveness of the intervention. Where more attractive interventions are compared with less attractive ones, any difference could therefore be a consequence of attractiveness and not its intrinsic worth. For health promotion interventions this is clearly important, but we cannot tell how important it is for therapeutic interventions without special studies to measure or refute such effects. These are difficult to do and are complex. Until the therapeutic effects of preference itself are more clearly understood, understanding the true therapeutic effects will be compromised, at least in principle.