The National Institute for Health and Care Excellence (NICE) invited Teva, the company manufacturing arsenic trioxide (ATO; tradename Trisenox®), to submit evidence for the clinical and cost effectiveness of ATO for untreated and relapsed or refractory acute promyelocytic leukaemia (APL). Kleijnen Systematic Reviews Ltd (KSR), in collaboration with Maastricht University Medical Center, was commissioned as the independent Evidence Review Group (ERG). This paper presents a summary of the company submission (CS), the ERG’s critical review of the clinical and cost effectiveness evidence in the CS, key methodological considerations and the development of the NICE guidance by the Appraisal Committee (AC). The CS presented three randomized controlled trials (RCTs). Two of these were trials in newly diagnosed APL (APL0406 and AML17) and the third trial was in patients with relapsed APL. Results from APL0406 showed that more people having AATO [ATO plus all-trans retinoic acid (ATRA)] were alive at 50 months compared with people having AIDA (ATRA in combination with idarubicin) (99% vs. 93%; p = 0.007). There was also a statistically significant lower cumulative incidence of relapse with AATO compared with AIDA at 50 months (2% vs. 14%; p = 0.001). At 4 years, results from AML17 showed a significant difference in event-free survival (91% vs. 70%; p = 0.002) favouring AATO but not in overall survival (93% vs. 89%; p = 0.250). The only trial presented for relapsed/refractory patients compared AATO with ATO, which was not a relevant comparison according to the NICE scope. The AC concluded that AATO was effective for untreated APL while for relapsed or refractory APL the effectiveness of ATO was considered uncertain and the long-term safety remains unexplored. In the CS base-case, AATO was less expensive (£31,088 saved) and more effective (2.546 quality-adjusted life-years (QALYs) gained) than AIDA and thus the dominating strategy for newly diagnosed low- to intermediate-risk APL. However, the ERG’s critical assessment highlighted a number of concerns, including deviations from the NICE reference case and a lack of detailed description and justification of parameters and assumptions related to (the extrapolation of) treatment effectiveness. However, it was reassuring that AATO for untreated APL remained dominant in the ERG base-case, and that the worst-case scenario produced by the ERG resulted in an incremental cost-effectiveness ratio (ICER) of £21,622. The AC concluded that although there was uncertainty in the model, it could recommend ATO for both untreated and relapsed or refractory APL.