The National Institute for Health and Care Excellence (NICE) invited the manufacturer of trastuzumab emtansine (T-DM1) (Kadcyla(®); Roche) to submit evidence of its clinical and cost-effectiveness for treating human epidermal growth factor receptor 2 (HER2)-positive, unresectable, locally advanced or metastatic breast cancer after treatment with trastuzumab and a taxane. The School of Health and Related Research Technology Appraisal Group (ScHARR-TAG) at the University of Sheffield were the independent Evidence Review Group (ERG) who produced a critical review of the company's submission to NICE. The ERG also independently searched for relevant evidence and modified the submitted decision analytic model to produce a revised estimate of cost-effectiveness and examine the impact of altering some of the key assumptions. The clinical effectiveness data were taken from two randomised controlled trials that reported a significant advantage in progression-free survival (PFS) for T-DM1 over lapatinib in combination with capecitabine (EMILIA trial), and over the treatment of physician's choice (TH3RESA trial). A network meta-analysis suggested T-DM1 was the best treatment in terms of both overall survival and PFS compared with lapatinib in combination with capecitabine; trastuzumab in combination with capecitabine; and capecitabine monotherapy. Adverse event (AE) data were taken from a pooled analysis of additional trials of T-DM1 as a single agent. The most common grade 3 or greater AEs for T-DM1 were thrombocytopenia and hepatotoxicity. Following the clarification process, the manufacturer reported a deterministic incremental cost-effectiveness ratio (ICER) for T-DM1 compared with lapatinib in combination with capecitabine of £167,236, the latter of which was estimated to have an ICER of £49,798 compared with capecitabine monotherapy. The ERG produced similar values of £166,429 and £50,620 respectively. All other comparators were dominated. During the appraisal, the manufacturer offered an analysis of a patient access scheme (PAS), which suggested that T-DM1 had a 0 % probability of being cost-effective at an ICER of £30,000 per QALY gained. The NICE Appraisal Committee concluded that while the clinical effectiveness of T-DM1 had been proven, it was not likely to represent a cost-effective use of National Health Service resources and therefore its use could not be recommended.