Homovanillic acid (HVA) and 5-hydroxy indole acetic acid (5-HIAA) concentrations have been estimated in the cerebrospinal fluid (CSF) of 16 and 18 patients respectively with Parkinsonism both before and during treatment with l-dopa. The rise of HVA correlated with dose of l-dopa. An increase of HVA concentration to less than 0·10 μg/ml. was associated with little or no clinical improvement. The five patients with the greatest increase of HVA concentration/g l-dopa showed little or no improvement from treatment and four of these patients had normal pre-treatment HVA and 5-HIAA. In those patients who responded well to l-dopa the CSF HVA/g l-dopa was intermediate between that of these two groups. It is suggested that change in HVA content of CSF during l-dopa treatment might be of value in the prediction of response to l-dopa.