The authors have analyzed the diagnostic problems presented by 100 cases of primary tumors of the filum terminale internum and conus medullaris. The final diagnosis was made either at operation (99 cases) or at autopsy (1 case). The most frequent type was ependymoma (88 cases), and 80 of these involved the filum terminale internum. Of the total of 100 tumors, 52 involved the filum alone, 17 involved the conus alone, and 31 involved both structures. Initial diagnosis was difficult because the disturbances of both sensory and motor function so frequently resembled those of other conditions. Pain in the back was present in 91 cases, but it was vaguely localized, sometimes appeared to have been precipitated by trauma, and was not constant. The interval between onset of symptoms and surgical treatment was therefore about 3.2 years. In avoiding diagnostic errors, examination of the spinal fluid and lumbar myelography with air or a contrast medium are important.