We present a patient in whom magnetoencephalograms were successfully performed in presurgical and postsurgical evaluations. A 12-year-old boy with congenital porencephaly was admitted with refractory adversive seizures and frontal absence seizures. Ictal magnetoencephalographic dipoles with frontal absence seizures were located in the left frontal lobe, anterior to the porencephalic cyst, and concordant with the same area detected by intraoperative electrocorticography. A partial cortical excision was performed, and the patient's cranial bone flap was removed because of an epidural abscess. The frontal absences then disappeared. The magnetoencephalogram revealed that secondary bilateral synchrony of focal discharges from the lesion may have caused the generalized seizures in this patient. Because of residual partial seizures, second and third magnetoencephalograms were performed, and we detected residual spike dipoles clustering in the area posterior to the cavity of cortical excision and anterior to the porencephalic cyst. Another excision of the area between the cavity and frontal edge of the cyst was performed, and seizure frequency diminished dramatically. In this case, despite the failure of dipole estimation by electroencephalogram in the context of a large bone defect, the magnetoencephalogram was useful in detecting the residual epileptogenic zone after failed epilepsy surgery.