A soluble form of the intercellular adhesion molecule-1 (sICAM-1) was measured in paired cerebrospinal fluid (CSF)/blood samples from 123 patients with different neurological diseases. Mean levels of circulating ICAM-1 in the blood were mean +/- SD = 423 +/- 184.6 ng ml-1 (range 44-1115 ng ml-1). Considerable differences of sICAM-1 in the CSF of patients were observed between disease groups. In acute bacterial meningitis, sICAM-1 levels as high as 1/5 of the serum concentration were detected in the CSF (n = 24; mean +/- SD = 33.0 +/- 23.7 ng ml-1; range: 4.8-93.9 ng ml-1). These changes coincided with a severe blood-CSF barrier dysfunction as indicated by a high CSF/blood ratio for albumin (mean +/- SD = 46.7 +/- 52.2; range: 16.8-249.3). In patients with polyradiculitis (n = 9; mean +/- SD = 14.5 +/- 11.9 ng ml-1; range: 2.6-43.7 ng ml-1) a similar covariation between the albumin and sICAM CSF/blood ratios was detected. In patients with multiple sclerosis (n = 9; mean +/- SD = 5 +/- 4.3; range: 0-12.7 ng ml-1) or HIV infection with neurological symptoms (n = 18; mean +/- SD = 4.9 +/- 3.2; range; 1-11.9 ng ml-1) low levels of sICAM-1 were detected in the CSF associated with intact blood-CSF barrier function in most patients. Among 13 patients with viral meningitis, only four had detectable levels of sICAM-1 in their CSF (mean +/- SD = 1.0 +/- 1.5 ng ml-1; range: 0-3.7).(ABSTRACT TRUNCATED AT 250 WORDS)