The tumor lysis syndrome has been described as biochemical disturbances associated with rapid destruction of tumor cells and subsequent synchronized massive release of cellular breakdown products sufficient to overwhelm excretory mechanisms and the body's normal reutilization capacity. The cardinal signs of the tumor lysis syndrome are hyperkalemia, hyperphosphatemia, hypocalcemia and hyperuricemia. Gefitinib (Iressa) is an oral, selective epidermal growth factor receptor (EGFR) inhibitor that has activity in female, non-smoker and non-small cell lung cancer with an EGFR mutation. Gefitinib is a well tolerated drug with few side effects. It has been associated with skin rash, diarrhea, nausea, a decrease in liver function and interstitial lung disease. However, there is no prior report of the tumor lysis syndrome associated with gefitinib. We report a case of a 54 year-old woman who developed tumor lysis syndrome that might have been induced by gefitinib after the treatment of adenocarcinoma of lung with an EGFR mutation.