This report details the histories of five patients with clinical diencephalic syndrome who collectively demonstrate the variability found in the syndrome with respect to: (1) clinical course, (2) site of the tumor, and (3) ease of obtaining radiologic confirmation of the presence of a tumor. A review of an additional 67 patients indicates that the observations are not unique. The anatomic variability combined with the fact that the course of those who are treated is infinitely better than those left untreated adds urgency to the establishment of precise anatomic diagnosis. These considerations led to a critical review of the histories of the 72 patients. From this it can be stated that anteriorly and posteriorly placed tumors do exhibit subtle but significant differences in their clinical course, and roentgenograms of the optic foramina and analysis of the CSF cell and protein content appear warranted early in the investigation of emaciation from unknown cause. Further, an evaluation is made of the role of various radiologic techniques and of endocrine studies in establishing the diagnosis. Similarly, the relative merits of radiotherapy and/or surgery in the treatment of the disease are defined. Finally, the adequacy of the term diencephalic syndrome is discussed.