There are dramatic changes in the electroencephalogram of the inactivated hemisphere in the intracarotid sodium amobarbital test. One of the more profound behavioral changes during this procedure is left hemispatial neglect accompanying right hemisphere inactivation. The present study was designed to ascertain whether there was a clear relationship between the degree of hemispheric inactivation (as measured by the electroencephalogram) and the degree of left hemispatial neglect during this procedure. Sixty-nine participants undergoing right hemisphere intracarotid sodium amobarbital testing were presented with a random letter cancellation test at various points during the procedure. Neglect was quantified as significant, moderate, minimal, or none, based on how many target letters the patients missed. The simultaneous electroencephalogram from each of these testing points was spectrally analyzed and topographic maps were generated. The degree of neglect was then compared with the comparable topographic map. It was found that as the amobarbital-induced right hemispheric dysfunction regressed, the degree of neglect lessened in a systematic fashion, as did the profound electroencephalographic changes induced by the drug. Thus, there is a clear relation between the degree of hemispheric inactivation induced by the amobarbital and the degree of left hemispatial neglect. This relationship held regardless of side of hemispheric language dominance or epileptic focus. These results replicate previous findings that right hemisphere inactivation during the intracarotid sodium amobarbital test results in left hemispatial neglect. They extend these findings by clearly showing that neglect changes in a quantitative fashion (rather than being an all-or-none phenomenon) and further, show that there is a clear relationship between the severity of neglect and the degree of hemispheric dysfunction. (JINS, 1998, 4, 99–105.)