One hundred and nineteen isotopic (chlormedrin-Hg203) encephalograms, 57 of which were positive, were reviewed in conjunction with other neuroradiologic studies and operative and autopsy findings. An over-all accuracy with respect to primary intracranial neoplasms of 92 per cent was obtained. The diagnostic accuracy of isotopic encephalography appears to compare favorably with other neuro-radiological diagnostic procedures and is considered a valuable addition to these methods for determining the presence and location of intracranial neoplasms.