To which groups of patients can the results of clinical trials be applied? This question is often inappropriately answered by reference to the trial entry criteria. Instead, the benefit and harm (adverse events, discomfort of treatment, etc) of treatment could be assessed separately for individual patients. Patients at greatest risk of a disease will have the greatest net benefit as benefit to patients usually increases with risk while harm remains comparatively fixed. To assess net benefit, the relative risks should come from (a meta-analysis of) randomised trials; the risk in individual patients should come from multivariate risk equations derived from cohort studies. However, before making firm conclusions, the assumptions of fixed adverse effects and constant reduction in relative risk need to be checked.