Cortical spreading depolarizations (CSDs) are a pathologic mechanism occurring in patients with aneurysmal subarachnoid hemorrhage and may contribute to delayed cerebral ischemia. We conducted a pilot study to determine the durations of depolarizations as measured by the negative direct current shifts in electrocorticography. Cortical electrode strips were placed in six patients (aged 35-63 years, Fisher grade 4, World Federation of Neurosurgical Societies [WFNS] 3-4) with ruptured aneurysms treated by clip ligation. Full-band electrocorticography was performed by direct current amplification (g.USBamp, Guger Tec, Graz, Austria) with ±250-mV range, 24-bit digitization, and recording/display with a customized BCI2000 platform. We recorded 191 CSDs in 4 patients, and direct current shifts of CSD (n = 403) were measured at 20 electrodes. Amplitudes were 7.2 mV (median; quartiles 6.2, 7.9), and durations were 2 min 14 s (1:53, 2:45). Ten direct current shifts in two patients with delayed infarcts were longer than 10 min, ranging up to 28 min. Taken together with previous studies, results suggest a threshold of 3-3.5 min to distinguish a normally distributed class of short CSDs with spreading hyperemia from prolonged CSDs with initial spreading ischemia. Results further demonstrate the clinical feasibility of direct current electrocorticography to monitor CSDs and assess their role in the pathology and management of subarachnoid hemorrhage.