The transport of brain derived neurotrophic factor (BDNF) across the blood-brain barrier (BBB) is negligible in normal conditions. However, BDNF might pass through the BBB when BBB is disrupted by a pathological condition such as stroke. This migration of BDNF through the BBB might be important during post-ischemic phase since BDNF can exert a protective action in the site of lesion. This study aimed to investigate plasma levels of BDNF in the acute phase of stroke in humans. Serial venous blood samples were taken in ten patients with acute stroke at the admission to the Stroke Unit and on the following 4 days. In the same samples we also evaluated the plasma levels of S100beta protein, a marker of BBB disruption. There was no significant change in BDNF plasma levels in our patients, even in the presence of a pronounced BBB dysfunction, as demonstrated by a significant increase of S100beta protein concentrations at days 2 and 3 after stroke. Our data, though indirectly, suggest that there is no significant increase in endogenous extracellular BDNF after a stroke in humans.