OBJECTIVES: Estimation of cost-effectiveness of health technologies tends to focus on the time period at or around launch, to fulfil the growing requirements of reimbursement or market access agencies. This study reviews the factors which influence cost-effectiveness over time and demonstrates the temporal impact on cost-effectiveness using a number of case-studies. The implications for decision making and market access are discussed. METHODS: A review of the factors that may influence cost-effectiveness over time and methodological approaches used to address these was conducted. Earlier analytical frameworks of studies from the 1990s in the fields of motor airbags, implantable cardiac defibrillators, statins, renal dialysis and hearing aids were revisited to re-estimate the cost-effectiveness. For example, parameters of an economic evaluation conducted in 1990 for erythropoetin were updated to 2004 values using a recent systematic review of clinical evidence together with revised unit costs and expert clinical opinion for resource utilisation. RESULTS: For the majority of case-studies examined, there was a trend for the reduction in cost-effectiveness ratios over time – e.g. for erythropoetin, the base-case cost per QALY decreased ten-fold over a 14 year period (£216,906 to £21,547). Significant factors included unit costs, dosage, utility gains and revised discounted rates. CONCLUSIONS: The timing of economic evaluation is critical in the estimation of cost-effectiveness. Production of this evidence may often be the first time that the conceptual framework of economic analysis has been applied to the technology, despite suggestions that economic evaluation should be used iteratively throughout the product life-cycle. This study has demonstrated that whilst there is a need for economic evaluation results to be timely to aid decision-making (i.e. at or around launch), it is important that the analysis is updated and reviewed periodically to assess whether cost-effectiveness has changed sufficiently to justify modifying the original decision.