We studied the clinical features, pathology, and molecular genetics of a family (Mo) with an autosomal dominant disinhibition, frontal lobe dementia, parkinsonism, and amyotrophy. We examined seven affected members and gathered clinical information on another six. The mean onset was at age 45 years. Personality and behavioral changes (disinhibition, withdrawal, alcoholism, hyperphagia) were the first symptoms in twelve. There was early memory loss, anomia, and poor construction with preservation until late of orientation, speech, and calculations. All affected members examined had rigidity, bradykinesia, and postural instability. Mean duration to death was 13 years. We studied the neuropathology of six individuals, five of whom had been examined in life. There was atrophy and spongiform change in the frontotemporal cortex, and neuronal loss and gliosis in the substantia nigra and amygdala. Two individuals, including one with fasciculations and muscle wasting, had anterior horn cell loss. There were no Lewy bodies, neurofibrillary tangles, or amyloid plaques. We call this disorder the "disinhibition-dementia-parkinsonism-amyotrophy complex" (DDPAC), based on the clinical syndrome found in this family and linkage to chromosome 17.