To be or not to be: Future lives in economic evaluation.

Sometimes healthcare will affect the health of people living in the future, or their chance of coming into existence. Should such outcomes be valued in health-economic evaluation? Guidelines implicitly recommend their inclusion but this rule has counterintuitive implications and is not consistently applied in practice. We suggest making a distinction between "necessary" and "potential" future lives in Health Technology Assessment. Necessary lives will exist independent of our healthcare choices and should be included. Potential lives are choice-dependent and should be excluded. This rule offers intuitive solutions within the HTA framework, and it changes the cost-effectiveness of several interventions where necessary future lives are affected.