Gefitinib ("Iressa", ZD1839) is a selective epidermal growth factor receptor-tyrosine kinase inhibitor, which blocks signal transduction pathways implicated in proliferation and survival of cancer cells. However, in vitro and in vivo studies concerning cell growth after withdrawal of gefitinib are limited. To determine whether cancer cells would resume proliferation upon removal of gefitinib, an in vitro study was undertaken using 4 human non-small cell lung cancer cell lines. With immunocytochemical staining and Western blot analysis, we confirmed positive expression of EGFR in these cell lines. We next evaluated growth inhibition before and after withdrawal of gefitinib. After incubation with 0-100 microM gefitinib for 24 h, medium containing gefitinib was removed, followed by addition of fresh growth medium to cell cultures. MTT assays were performed daily over a 6-day time course. Even at very low levels of gefitinib (<1 microM), these 4 TKB cells had 1-10% growth inhibition, respectively. With these levels of gefitinib, continued inhibitory effects after withdrawal of getitinib were observed in 3 of the 4 cell lines. Furthermore, none of the 4 cell lines, including the cell line which had abolished growth inhibition, showed accelerated re-growth rate after the withdrawal of gefitinib even in these exposure conditions. Based on our in vitro experiments, additional in vivo studies, which can compare the pre- and post-treatment growth rate, will be necessary to help better understand the mechanisms behind cell growth recovery after temporary gefitinib treatment.