Although it is well known that infectious diseases of the cerebrospinal leptomeninges are accompanied by abnormally low levels of the CSF sugar, there has been little recognition of the fact that diffuse neoplastic involvement of the leptomeninges may also result in this phenomenon. A total of 57 autopsy-proven cases of diffuse meningeal neoplasms with recorded CSF sugar levels have been collected from the literature and from the files of the Presbyterian Hospital and Neurological Institute of New York. In 43 of these cases (75%) the CSF sugar contents were below 40 mg.%, often as low as 5-10 mg.%. Any diffuse meningeal neoplasm[long dash]be it carcinoma, glioma, sarcoma, lymphoma, or melanoma[long dash]is likely to be accompanied by lowered sugar levels. Of the various mechanisms possibly responsible for this phenomenon, it is suggested that increased utilization of CSF sugar by the sheets of tumor cells is the most plausible explanation. This association carries implications of considerable diagnostic importance.