Stereotaxic surgery was performed in 27 patients. Complete elimination of or significant reductions in hyperkinesia were obtained in 17 cases; five patients died. There was no correlation between the severity of clinical manifestations of hepatocellular dystrophy and the relatively normal quantitative measures of cortical and subcortical biopotentials, which were produced on a background of microstructural changes affecting neurons in these regions. It is suggested that qualitative significance of these biopotentials is that they carry an excess pathological spike activity resulting in hyperkinesia. This is supported by the fact that hyperkinesia was suppressed after surgical destruction of the ventrolateral nucleus of the thalamus and subthalamic structures.