Choline concentrations were measured in lumbar cerebrospinal fluid (CSF) and plasma of 37 healthy normal subjects and 13 young (age range, 21 to 34 years) and 6 older (age, greater than or equal to 45 years) healthy adults with Down's syndrome (DS). All subjects with DS had a trisomy 21 karyotype, and 3 of the 6 older subjects were demented as judged from a history of mental deterioration, disorientation, and memory loss. In healthy normal subjects, there was a significant correlation between age and CSF choline concentrations. Compared with age-matched controls, in young subjects with DS, CSF, but not plasma, choline concentrations were elevated (by 49%), whereas in older subjects with DS, CSF and plasma choline concentrations were similar to control values. The CSF choline concentrations were unrelated to body height in the DS and control groups, and rostrocaudal CSF choline gradients did not differ between either the control and DS groups or the young and old subjects with DS, suggesting that the elevation in the CSF choline concentration in young subjects with DS was not related to their shorter stature. Since increased CSF choline concentrations in young adult subjects with DS were accompanied by normal plasma choline concentrations, these results suggest that young adults with DS have either an increased release of choline from the brain or a reduced rate of clearance of choline from CSF.