This is a summary of the evidence review group (ERG) report on the clinical effectiveness and cost-effectiveness of adjuvant imatinib post resection of KIT-positive gastrointestinal stromal tumours (GISTs) compared with resection only in patients at significant risk of relapse. The ERG report is based on the manufacturer's submission to the National Institute for Health and Clinical Excellence (NICE) as part of the single technology appraisal (STA) process. The bulk of the clinical evidence submitted was in the form of one randomised controlled trial (RCT), the Z9001 trial, funded by the manufacturer, which compared resection + adjuvant imatinib for 1 year to resection only. Results were immature, with median recurrence-free survival (RFS) not yet having been reached at the time of analysis. The trial did provide evidence of a delay in disease recurrence [1-year RFS rate of 98% in the imatinib arm vs 83% in the placebo arm [hazard ratio (HR) 0.35, 95% confidence interval (CI) 0.22 to 0.53, p < 0.0001)] but no evidence of an overall survival benefit. There was no long-term evidence around the rate of imatinib resistance over time with different treatment strategies (± adjuvant treatment). The relevant patient group for this appraisal is those at significant risk of relapse. These form a subgroup of the Z9001 trial, and all information regarding this group was designated 'Commercial-in-Confidence' (CIC). Median observation time for RFS was also CIC. The manufacturer constructed a Markov model comprising 10 health states designed to estimate costs and effects of treatment over a lifetime time horizon. The manufacturer's estimate of the base-case incremental cost-effectiveness ratio (ICER) was 22,937 pounds/quality-adjusted life-year (subsequently amended by the manufacturer to 23,601 pounds). While the structure of the model reasonably reflected the natural history of the disease, the ERG had numerous concerns regarding the selection of, and assumptions around, input parameters (utilities, monthly probabilities of recurrence and death). Furthermore, the model was set up in such a way that any delay in recurrence translated directly into a survival benefit, an assumption that has no evidence base. A further assumption not supported by evidence was that any treatment benefit gained in the first year is carried on for a further 2 years at the same rate. Appropriate probabilistic sensitivity analysis was undertaken on the base case only, but not on scenario analyses, or choice of model used to estimate long-term survival data. The model was not amenable to changes in input values, thus limiting any additional analyses by the ERG to test assumptions. Due to the large number of uncertainties and assumptions, the estimated ICERs should be regarded as highly uncertain. The guidance issued by NICE in June 2010 as a result of the STA does not recommend imatinib as adjuvant treatment after resection of gastrointestinal stromal tumours, although individuals currently receiving adjuvant imatinib should have the option to continue treatment until they and their clinician consider it appropriate to stop.