The accurate localization of primary language cortex is one of the goals in the evaluation of brain surgery candidates. In this paper we describe the localization of expressive language cortex using magnetic source imaging (MSI) in a patient with refractory epilepsy caused by a tumor affecting the left inferior frontal gyrus. The magnetoencephalographic recordings during an expressive language task were co-registered with a magnetic resonance imaging (MRI) scan obtained after subdural grid placement and provided a cluster of sources of brain activation in the vicinity of the lesion. This map of expressive language provided by MSI was verified with electrocortical stimulation before the operation. No speech problems were reported in our patient after the resection of the lesion, suggesting that MSI is an accurate non-invasive method for the pre-surgical mapping of expressive language in cases where there is clear functional risk during tumor resection.