STI571, or imatinib, selectively inhibits BCR/ABL, PDGFR and c‐kit kinase activity. It has been reported that a large proportion of small cell lung cancer (SCLC) cell lines and tumors express c‐kit and that STI571 inhibits tumor cell growth. We therefore investigated the therapeutic efficacy of STI571, alone or combined with chemotherapy, in human SCLC cells or tumors xenografted into nude mice. The level of c‐kit mRNA expression was variable in SCLC tumors (positive for 2 of 4 xenografts), and c‐kit protein was not detected by immunohistochemistry. On the 4 xenografted tumors, PDGFRα and PDGFRβ were not detected by immunohistochemistry. STI571 induced inhibition of proliferation of the SCLC6 cell line without inducing apoptosis; in contrast, in combination with etoposide or topotecan, the growth inhibition of SCLC6 cells induced by STI571 was increased, with apoptotic DNA fragmentation. Four human SCLC xenografts (SCLC6, SCLC61, SCLC74 and SCLC108) were transplanted into mice. After intraperitoneal injection of STI571, we observed 80%, 40% and 78% growth inhibition of SCLC6, SCLC61 and SCLC108 tumors, respectively, without any significant inhibition of SCLC74 tumor growth. In mice bearing responsive SCLC tumors, we observed an increase of growth inhibition induced by chemotherapy (etoposide + ifosfamide or topotecan) by concomitant and continuous administration of STI571, associated with an increase of toxic deaths. In SCLC6‐bearing mice receiving sequential treatments, we observed a reduction of toxic deaths but a decrease of synergistic antitumor efficacy. In conclusion, the efficacy of STI571 alone in SCLC xenografted tumors was variable and did not depend on c‐kit expression. Moreover, a significant increase of chemotherapy‐induced growth inhibition was obtained by concomitant administration of STI571 that should be carefully investigated in SCLC patients. © 2004 Wiley‐Liss, Inc.