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Matrix metalloproteinase-9 (MMP-9) was investigated by enzyme-linked immunosorbent assay (ELISA) and zymography in 111 paired CSF and serum samples from patients with various neurological disorders. In 20 patients with blood-brain barrier (BBB) impairment but normal CSF cell count, elevated levels of MMP-9 were not observed by ELISA measurement. Another 11 patients characterized in the same way, exhibited only slightly increased MMP-9 levels. In contrast, in 12 patients with intact BBB but elevated CSF cell count, MMP-9 was increased too. It was shown by the more sensitive zymography that MMP-9 increased if CSF cell count exceeded five cells per microl. Spearman rank statistics revealed that MMP-9 concentration in CSF correlated with CSF cell count (r=0.755; P<0.0001), but not with CSF/serum albumin ratio (Q(Alb)) (r=0.212; P=0.057), a measure for BBB impairment. Moreover, the CSF/serum MMP-9 ratio (Q(MMP-9)) did not correlate with Q(Alb)(r=0.192; P=0.100). By use of a Boyden chamber, in which granulocytes migrated through a reconstituted basement membrane, it was demonstrated that the MMP-9 concentration in the lower chamber correlated very significantly with the number of accumulated cells (r(2)=0.7692; P<0.0001). The meaning of the increase of MMP-9 in CSF is critically discussed.