OBJECTIVES:  Phase III, randomised controlled trials remain the gold standard for health technology assessment (HTA) submissions. Data sets may be supplemented with other sources (e.g. Phase II trials, observational studies, mixed treatment comparisons). However, the influence of expanded data sets on HTA appraisals is unclear. METHODS:  We reviewed recent National Institute for Health and Care Excellence (NICE; England and Wales) oncology drug submissions to determine the frequency and type of expanded data sets. We then evaluated the influence of expanded data on agency decisions. A similar review of submissions to the Australian Pharmaceutical Benefits Advisory Committee (PBAC) was performed. RESULTS: There were 30 relevant appraisals on the NICE website covering a range of cancer types. Of these, 14/30 made use of expanded data sets featuring Phase II trials, observational studies, meta-analyses and/or mixed treatment comparisons among other sources. Reasons for using expanded data sets included: agency concerns over Phase III studies, lack of long-term or head-to-head data and limited Phase III data. Where additional data were included, around one third (5/14 cases, 35.7%; 5/30 [16.7%] overall) appeared to have directly influenced the final decision. Overall, positive appraisals were less frequent for submissions that featured expanded data sets compared with submissions featuring Phase III data only (2/14 [14.3%] versus 9/16 [56.3%]). Comparable to NICE, 2/10 (20%) of PBAC submissions were influence by expanded data sets, although cost-effectiveness data were crucial for PBAC approval overall.  CONCLUSIONS: We found that expanded data sets feature in nearly half of recent NICE oncology HTA assessments. However, additional data appeared to influence only one in five appraisals by NICE and PBAC. Expanded data sets have a place in contributing to HTA decision making, but overall, rigorous Phase III RCT data remain essential to obtaining a positive HTA appraisal.