Twenty years after the epidermal growth factor receptor (EGFR) was identified as a potential anticancer target, the EGFR inhibitor gefitinib (Iressa; AstraZeneca) has been approved for the treatment of patients with advanced non-small-cell lung cancer in many countries. Studies have indicated its potential for treating patients with other types of solid tumours. Investigation of gefitinib has not only increased our knowledge about the biology of EGFR signalling, but is contributing to our evolving understanding of which tumours are EGFR dependent.