A type of pathological behavior, imitation behavior (IB), is newly described. In this behavior patients imitate the examiner's gestures, although not instructed to do so. Patients explain that they thought they had to imitate the examiner. IB is the first stage of utilization behavior (UB). Neuropsychological examination of 40 patients with IB, of 35 with UB, and of 50 disease controls demonstrates the existence of a frontal syndrome and two determining features of such behavior: dependence on (1) the social and (2) the physical environments. Loss of intellectual control was also required for the occurrence of such behavior. UB and/or IB were present in 96% of the 29 patients with focal lesions of the frontal lobes. Computed tomographic scans in 26 of these patients showed involvement of the inferior half of the anterior part of one or both frontal lobes. IB and UB are interpreted as release of parietal lobe activities, resulting from impairment of frontal lobe inhibition.