We compared the anesthetic effects of desflurane and isoflurane using percent burst suppression of the electroencephalogram (EEG) as an end-point in 10 neurosurgical patients. The EEG was recorded from frontal leads and processed variables were analyzed as a function of increasing isoflurane and desflurane concentration with age and baseline delta EEG power (0.5-3.75 Hz) as independent variables. Isoflurane and desflurane (0.5, 1.0, 1.5, 2.0 minimum alveolar anesthetic concentration [MAC]) were incrementally administered until the EEG was quiesecent at least 40% of the time. Both anesthetics were evaluated separately in each patient. By analysis of variance, isoflurane and desflurane produced dose-related increments in burst suppression which were significantly affected by the age and baseline delta EEG of the patient. When isoflurane and desflurane were equated by MAC and adjusted for age, they produced statistically similar patterns of EEG burst suppression. Within subjects, a high degree of correlation was observed for percent burst suppression between equipotent levels of isoflurane and desflurane (r = 0.85; P < 0.05). Patients with baseline delta EEG power less than 80% of total power showed increases in delta EEG and decreases in median frequency with isoflurane and desflurane. Patients with baseline delta EEG power > 80% of total power produced no change in EEG frequency with increasing anesthesia but revealed a greater sensitivity to the development of burst suppression. These results show that isoflurane and desflurane produce similar EEG suppression in neurosurgical patients. If the EEG is initially slow, further slowing cannot be used to assess anesthetic depth.