During the treatment of four patients with cerebral manifestation of Wilson's disease, we measured the copper concentration in the cerebrospinal fluid (CSF) and serum, the: serum coeruloplasmin concentration, the free copper concentration in the serum, and the albumin ratio CSF/serum (AR). These measurements were treated as indicators of the copper-related toxic effects on the brain and the blood-brain barrier (BBB). The half - life of the decrease in the CSF copper concentration during therapy was 23.5 +/- 5.78 months (mean +/- S.E.M.). The therapeutic - target - copper concentration in the CSF (mean normal concentration) is below 20 mu g/l. The average length of therapy needed to normalize (CSF - copper values in our patients with an average initial value of 76.25 mu g/L was 47 month. During the first 10 month of treatment there was an increase in all cases of the measured disturbance in the blood-brain barrier (measured as the ratio of albumin in CSF to albumin in serum, AR). All patients showed an initial worsening of the neurological condition, on average after 1.75 +/- 0.25 months. The maximal rise in AR, from the initial values, was on average 18.4 +/- 5.08%; this maximum was reached after an average of 6.9 +/- 1.5 months. The AR normalized during therapy, indicating a reduction in toxicity in the blood-brain barrier region. The extent of the AR increases in individual patients did not correlate significantly with CSF copper half-life, serum copper half-life, the initial half - life of the reduction in the ratio (copper in serum)/(coeruloplasmin in serum), the initial copper concentration in CSF or serum, the initial free copper concentration in serum, or the initial dose of penicillamine (within the first 2 months). We conclude that the normalization of the CSF copper concentration in patients with the cerebral manifestation of Wilson's disease is a slow process, even if therapy is sufficient. The initial worsening of the neurological condition which has often been reported may be reflected in the disturbance of blood-brain barrier function, which we have measured here for the first time (using the parameter of the albumin ratio CSF/serum). Based on repeated measurements of the AR during the course of treatment it seems that the brain toxicity of mobilized copper can be assessed and the therapy adjusted.