We examined the impairment of consciousness during partial seizures (PS) arising from various brain sites according to the operational definition of the international classification, i.e., altered awareness and/or responsiveness. The subjects were 142 patients who underwent intracranial EEG evaluation and subsequent resective surgery. First, the patients were examined to determine whether they usually had been partially or completely aware of their seizures. Second, spontaneous habitual seizures that had been videotaped with simultaneous intracranial EEG recording were reviewed to determine responsiveness and recall during ictal behavioral alterations. In all, 114 patients were partially or completely aware of their seizures. Patients who tended not to be aware of their seizures were those with frontal lobe epilepsy (FLE) with extensive epileptogenic regions on the language nondominant side and those with temporal lobe epilepsy (TLE) with seizure origin in the lateral cortex of the language dominant side. Of the 21 patients with FLE, 88 with TLE, and 4 with occipital lobe epilepsy, 7, 22, and 2 patients responded to stimuli during the seizure, respectively, but only 11 of the patients with FLE and none of the other patients could recall the stimuli applied during the behavioral alterations. Bilateralization of seizure discharges correlated with impaired responsiveness. According to the International Classification, about half of patients with FLE had only simple partial seizures (SPS) and the other patients had complex partial seizures (CPS). Altered awareness and/or responsiveness occurred in most habitual partial seizures in our subjects. The term "complex" appears to be useful in clinical practice, although the contents of ictal behavior and the site or side of seizure origin are not implied.