ZD1839 ('Iressa') is an orally active, selective epidermal growth factor receptor-tyrosine kinase inhibitor which blocks signal transduction pathways implicated in the proliferation and survival of cancer cells and other host-dependent processes promoting cancer growth. In preclinical studies, ZD1839 produced reversible growth inhibition and growth delay in a wide range of tumour cell lines and human tumour xenografts. Moreover, this activity was enhanced when ZD1839 was coadministered with cytotoxic agents. Preliminary results from phase I trials in patients with advanced disease and a wide variety of tumour types suggest that ZD1839 has an acceptable tolerability profile and promising clinical efficacy, particularly in non-small cell lung cancer (NSCLC). ZD1839 is currently in phase III clinical development for the treatment of advanced NSCLC. In addition, further trials are ongoing or planned in a number of other tumour types.