We examined first evidence for the significance of SALL4, a transcription factor essential for embryonic development and the self-renewal of embryonic stem (ES) cells, as a natural resistance factor against anticancer drugs in lung cancer. To determine the significance of SALL4 expression in lung cancer cells, small inhibitory RNA (siRNA) against SALL4 was transduced into A549 and SBC-3 cells, resulting in increased sensitivity towards anticancer drugs [cisplatin (CDDP), carboplatin (CBDCA), and paclitaxel (PTX)]. SALL4 cancer tissues from 31 lung cancer patients were used to assess clinical significance. The analysis showed differences in SALL4 expression corresponding to the therapeutic outcomes. SALL4 expression measured before adjuvant chemotherapy was significantly higher in the patients showing recurrence of cancer than in the disease-free patients. In addition, the period until recurrence was shorter in the patients showing high SALL4 expression. These results indicate that SALL4 overexpression acts as a natural resistance factor and may be involved in the recurrence of lung cancer after adjuvant chemotherapy.