The patients with Parkinson’s syndrome and multiple sclerosis show lower levels of HVA in CSF than most other patients. The latter group also has low levels of 5-HIAA. Hydrocephalus is generally supposed to have high 5-HIAA. Our study with normotensive hydrocephalus shows that these patients have normal levels. Decrease in HVA is found in spinal cord lesions with restricted flow, as has been pointed out by Curzon et al. Low values of 5-HIAA are seen in patients with transverse lesions presumably due to disappearance of the spinal portion of 5-HIA