Gefitinib is currently one of the mostly used epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) recommended for treating nonsmall cell lung cancer. However, drug resistance is observed among the majority of patients after initial treatment. Factors that predict treatment prognosis and drug resistance to EGFR-TKIs remain elusive. The objective of this study is to investigate whether leukocyte relative telomere length (RTL) can be used as a prognostic biomarker of EGFR-TKIs therapy. In this study, 369 patients with stage IIIB or IV lung adenocarcinoma were recruited and treated with gefitinib as first-line monotherapy. Leukocyte RTL of each patient was measured using quantitative polymerase chain reaction protocol and calculated according to Cawthon's formula. Finally, we examined the association between leukocyte RTL and prognosis or drug resistance of advanced lung adenocarcinoma to gefitinib treatment. Our results indicated that compared with long RTL, short leukocyte RTL was significantly associated with poor prognosis in all patients after gefitinib treatment (overall survival [OS]: 12.9 months vs. 17.8 months, p=1.2x10(-4); progression-free survival: 7.8 months vs. 13.0 months, p=0.043). In addition, statistically significant association between short leukocyte RTL and short OS still existed among the EGFR mutant patients (hazards ratio [HR]=1.65, 95% confidence interval [CI]=1.28-2.12; p=0.006). Besides EGFR mutation status, short RTL also contributed to remarkably elevated risk of gefitinib primary resistance (HR=1.50, 95% CI=1.05-2.15, p=0.027). Our results highlight the clinical potential of leukocyte RTL as a novel biomarker in advanced lung adenocarcinoma treated with EGFR-TKIs.