Instrumental variable analysis is a recently propagated method to deal with confounding and to estimate therapeutic effects in observational studies. An instrumental variable is a factor which affects treatment but is not related to patient prognosis. A theoretical advantage of a well-chosen instrumental variable is that both measured and unmeasured confounders do not influence the effect estimator. Examples of instrumental variables previously used are regional differences in treatment and physician prescribing preference. Application of instrumental variable analysis seems most suited to large patient registries with considerable expected residual confounding in case standard analytical methods are applied.