The relationship between the pharmacokinetics and long-term antitumor activity of gefitinib in patients with epidermal growth factor receptor(EGFR)mutation-positive lung adenocarcinoma has not yet been clarified in clinical practice. The present study assessed the correlation between the pharmacokinetics and long-term therapeutic effects of gefitinib in patients with lung adenocarcinoma harboring the EGFR-activating mutation.Fifteen patients with lung adenocarcinoma harboring the EGFR mutation were administered 250 mg of gefitinib daily. Blood samples were collected prior to the first administration of gefitinib and after 1, 4, 6, 8, and 24 h. Plasma concentrations of gefitinib were measured via liquid chromatography mass spectrometry, and the peak plasma concentration(Cmax)and area under the plasma concentration time curve from 0 to 24 h(AUC 0-24)of gefitinib were determined. The correlations between these pharmacokinetic variables and the objective responses, including progression-free survival(PFS)and overall survival(OS), were retrospectively analyzed.The Cmax of gefitinib in patients with a partial response(PR)was significantly lower than that of patients with stable disease(SD)(median Cmax: 278 vs 588 ng/mL, p<0.05 ). However, the Cmax of gefitinib did not correlate with longer PFS. Conversely, a significant negative correlation was found between the AUC 0-24 of gefitinib and longer survival(r=-0.545, p<0.05 ).It may be possible that a high concentration of gefitinib is not necessary to achieve long-term therapeutic effects in patients with lung adenocarcinoma harboring the EGFR mutation.