OBJECTIVES: Within health economic models of metastatic cancer therapies assumptions on the relationship between progression-free survival (PFS)/ time-to-progression (TTP) and overall survival (OS) are typically required; notably when OS data are immature or unavailable. A review was undertaken to identify the methods that have been used within health economic models regarding this relationship and to identify the rationale given for the approach taken, specifically in those situations where OS data were not available or immature. METHODS: All NICE technology appraisals in the advanced and/or metastatic cancer setting completed by December 2013 were reviewed. The review included all relevant appraisal documents publicly available on the NICE website containing information on the methods used and/or rationale for the approach taken to model the relationship between OS and PFS/TPP within the health economic model. This included the sponsor submission and updated analyses, the independent Assessment Report, and other reports/analyses in relation to the appraisal process.  RESULTS: In those instances where OS data were immature or not available, PFS/TTP was typically assumed to be a valid surrogate of OS. Justification for this assumption was inconsistently reported. In some health economic models a quantification of the assumed relationship was informed by published evidence and/or expert judgement. In some cases attempts were made to explore the potential impact of this relationship in sensitivity analysis. CONCLUSIONS: The methods and/or rationale given for the approach used to model the relationship between OS and PFS/TTP in health economic models has been inconsistently reported and justified. Whilst some health economic models attempted to quantify this relationship, further transparency is required. A consensus needs to emerge on the most appropriate approaches to be used within health economic models to quantify this relationship, specifically when OS data are not available or immature and to identify the circumstances when particular approaches may be most relevant.