Gefitinib, an epidermal growth factor receptor tyrosine kinase inhibitor, has been reported to have a certain anti-tumor effect in previously treated patients with non-small cell lung cancer (NSCLC). However, the prognostic factors in those patients with and without a positive response to gefitinib treatment remain unclear. A retrospective chart review was performed in 131 advanced NSCLC patients who received 250 mg of gefitinib as either a second-line or even later stage treatment from July 2002 to December 2005. The clinical factors including age, gender, performance status (PS), stage, histology, the number of prior types of chemotherapy, and the response to first-line chemotherapy were analyzed. One and 38 patients experienced a complete and partial response, respectively, to gefitinib treatment with an overall response rate of 30%. The median survival time (MST) of all patients receiving gefitinib treatment was 10 months while the MST was 28 months in the 39 gefitinib responders and 6 months in the 92 non-responders. Among the 39 gefitinib responders, the predominant prognostic factor was found to be the effectiveness of the first-line chemotherapy. The MST of the 20 patients with a response to the first-line chemotherapy was 32 months while the MST of the 19 patients without a response to the chemotherapy was 22 months (p=0.025). Among the 92 gefitinib non-responders, the predominant prognostic factor was the PS (p<0.001). The effectiveness of the first-line chemotherapy was therefore found to be a prognostic factor in the gefitinib responders with previously treated NSCLC, while the PS was shown to be a prognostic factor in the gefitinib non-responders.