Neutropenia (a reduction in white blood cells) is a rare, but potentially serious side effect of the cancer drug gefitinib (Iressa). Iressa is used to treat non-small cell lung cancer (NSCLC) with mutations in the EGFR gene. A patient with EGFR-mutant advanced adenocarcinoma of the lung (a type of NSCLC) was treated with Iressa. Her tumor shrank, but she experienced severe neutropenia, leaving her at risk of dangerous infections. She was switched to erlotinib (Tarceva), another EGFR inhibitor, after which her neutropenia cleared up. The patient has since continued on Tarceva without neutropenia or cancer progression for over 9 months. This case suggests that Iressa-induced neutropenia can be safely treated by switching to Tarceva, although caution should be used in drawing conclusions from a single case study.