Interictal high-frequency oscillations (HFO) were recently identified in recordings from depth macroelectrodes in epileptic patients. StereoEEG (SEEG) recordings were analyzed in four patients with medically intractable partial seizures due to focal cortical dysplasia type IIA. Characteristics of HFO within seizure onset zone (SOZ), irritative zone, and remote brain areas were investigated. Whilst the rate of occurrence for ripples (80-200 Hz) was significantly higher in recordings from within than outside the SOZ, the rate of fast ripples (200-450 Hz) was less reliable index of SOZ. Interestingly, the mean powers across subjects were significantly higher within than outside the SOZ in both ripple and fast ripple frequency ranges. Our study demonstrates a capacity of interictal HFO to detect the SOZ in focal cortical dysplasias.