The purpose of this study was to predict chemotherapy response by technetium-99m methoxyisobutylisonitrile (Tc-99m MIBI) lung single-photon-emission computed tomography (SPECT) and compare P-glycoprotein (Pgp) expression in patients with untreated small cell lung cancer (SCLC). Before chemotherapy, 40 patients with untreated SCLC underwent Tc-99m MIBI lung SPECT. Immunohistochemical analyses were performed using multiple nonconsecutive sections of the biopsy specimens to detect Pgp expression. Chemotherapy response was evaluated in the third month after completion of treatment by clinical and radiological methods. Based on quantitative analyses, the tumor uptake ratios (TUR) of the 20 patients with good response (1.89 +/- 0.28) were significantly higher than that of the 20 patients with poor response (1.21 +/- 0.28) (p value < 0.05). Based on visual interpretation, all of the 20 patients (100%) with good response had positive Tc-99m TF lung SPECT findings and negative Pgp expression. Five of the other 20 patients (25%) with poor response had positive Tc-99m MIBI lung SPECT findings, and 12 of the other 20 patients (60%) with poor response had negative Pgp expression (p value < 0.05). Negative Tc-99m MIBI lung SPECT findings could predict poor response. Therefore, we concluded that Tc-99m MIBI lung SPECT can accurately predict the chemotherapy response, and Tc-99m MIBI lung SPECT findings can be partially compatible with Pgp expression in patients with untreated SCLC.