OBJECTIVES: Budget impact models (BIMs) which demonstrate the economic impact of introducing or increasing the use of specific treatments are routinely used to assist the NHS with financial planning. A core component of any BIM is the estimation of the eligible patient population. The objective of this study was to identify an appropriate methodology for estimating the size of the stage IIIb/IV EGFR M+ non-small cell lung cancer (NSCLC) patient population eligible for first-line treatment with a tyrosine kinase inhibitor such as afatinib (GIOTRIF®). METHODS: A review of the approach taken by NICE in the costing statements of all treatment options for patients with advanced (stage IIIb) or metastatic (stage IV) EGFR M+ NSCLC was conducted. The costing statements of tyrosine kinase inhibitors afatinib, erlotinib and gefitinib were reviewed, as was the costing statement for the chemotherapy agent pemetrexed. RESULTS: Based on the reviewed approaches, the calculation can be broken down into six discrete steps from the estimation of the general population to the target population: (1) Incidence of lung cancer; (2) Proportion of NSCLC; (3) Proportion with stage IIIb/IV NSCLC; (4) Proportion who receive first-line chemotherapy; (5) Proportion with EGFR mutation status; and (6) Proportion who are EGFR M+. A detailed breakdown of the methods used to calculate the patient population eligible for treatment with afatinib was not available in the respective NICE costing statement; however the eligible population estimated by NICE validates that this approach is reasonable. CONCLUSIONS: The methodology employed by NICE to estimate the proportion of stage IIIb/IV EGFR M+ NSCLC patients was broadly consistent across all costing statements considered. Is it reasonable to assume that this approach, used to estimate the population of stage IIIb/IV EGFR M+ NSCLC patients in England and Wales is also applicable in Scotland and Ireland.