We present the rationale and study design of the CGMT (combined gefitinib and metformin therapy) trial (www.ClinicalTrials.gov Identifier: NCT01864681), which is aimed at treating locally advanced non–small-cell lung cancer. The CGMT trial is a multicenter, phase II randomized, double-blinded, and placebo-controlled study, which is designed to evaluate the safety and efficacy of metformin in combination with gefitinib as first-line therapy in patients presenting with stage IIIb-IV non–small-cell lung cancer expressing the epidermal growth factor receptor mutant. Two therapies are proposed for this trial. The first regimen is comprised of gefitinib plus metformin. The second therapy is comprised of gefitinib plus placebo. The primary objective of this trail is to compare the progression-free survival rate at year 1 of the study. The secondary objective of this trial is to compare the 2-year overall survival, the 2-year progression-free survival, the objective response rate, and the disease-control rate, and to evaluate the relative safety of both therapies. Based on the statistical design, we plan to enroll approximately 200 patients.