More than 90% of gastrointestinal stromal tumors (GISTs) express the receptor tyrosine kinase KIT, and activating mutations of the KIT gene are detectable in the vast majority of these tumors. Imatinib mesylate (formerly STI571) is a potent inhibitor of KIT kinase activity and has been proven to be highly active in patients with unresectable or metastatic GIST expressing immunohistochemically detectable KIT protein. Here we report a patient with metastatic GIST who responded well to imatinib mesylate treatment despite the near absence of KIT expression in two different samples of his tumor. The tumor was morphologically typical for a GIST, stained positively for CD34, and harbored an in-frame deletion (WK 557–558) in KIT exon 11 that is common in GISTs. Our experience with this patient suggests that even GISTs with very low levels of KIT expression may respond to imatinib mesylate therapy.