We studied levels of T3, T4, FT3, FT4, rT3 and TSH concentrations in serum and FT3, FT4, rT3 and TSH concentrations in cerebrospinal fluid (CSF) in 10 patients with brain tumor and 20 patients with acute stroke and compared them to 7 patients in control group (further clinical evaluation in control group did not show brain lesions). All patients were euthyroid. The study was approved by local Ethical Committee. Serum T3 and T4 levels were similar in all three group. The values of FT3, FT4 and TSH did not significantly differ to control group neither in serum nor in CSF. On the contrary, significantly elevated rT3 was found in serum and CSF, at both group of patients. The rT3/FT3 ratio were the highest in patients suffering from brain tumor and were significantly elevated compared to control group (serum, CSF), as well as compared to the patients with acute stroke. The values were particularly high in CSF (4 times higher) which would suggest that changes connected with low T3 syndrome in patients with brain lesion are more obvious in CSF than in serum and identify brain tumor as a prototype of serious local nonthyroid illness. Serum and CSF test showed positive correlation for FT4 and FT3 in patients with acute stroke and for rT3 in patients with brain tumor. This suggests that hormones are passing through still functional blood-brain barrier. The study did not show correlation between elevated rT3 or rT3/FT3 ratio and poor prognosis. Thyroid hormones are present in CSF at concentration lower than in serum. There are probably two mechanisms responsible: hormones are partly crossing the blood-brain barrier from serum, but also T3 and rT3 may derive from local conversion of T4 within the central nervous system. The impairment of this conversion which occurs in different brain lesions could be responsible for the changes in hormones level known as low T3 syndrome, which are particularly evident in CSF.