We measured soluble intercellular adhesion molecule-1 (sICAM-1) in the serum and cerebrospinal fluid (CSF) of 35 clinically active relapsing-remitting (RR) multiple sclerosis (MS) patients who underwent both lumbar puncture and gadopentetate dimeglumine (Gd-DTPA)-enhanced MRI within an interval of 1 week, and of 30 neurological controls of whom 17 had noninflammatory neurologic diseases (NIND), 8 bacterial meningitis (BM), and 5 AIDS dementia complex (ADC). Thirteen of the MS patients assumed corticosteroids at the time of the study. While sICAM-1 serum levels were highest in the BM group (p < 0.005), untreated MS patients showed levels higher (p < 0.05) than treated MS and NIND, but similar to ADC. Moreover, the untreated MS group had CSF/serum sICAM-1:CSF/serum albumin (sICAM-1 index) values higher than the treated group (p < 0.01), NIND (p < 0.005), and BM (p < 0.05); high sICAM-1 index was found also in ADC. Untreated MS patients with one or more Gd-DTPA-enhancing MRI lesions (Gd-positive) had higher mean values of CSF/serum albumin ratio (QAlbumin) and CSF mononuclear cells compared to patients without such lesions (Gd-negative). In the untreated Gd-negative patients, sICAM-1 serum levels were higher (p < 0.05) than those in Gd-positive patients. In the latter group, there were positive correlations between the number of CSF mononuclear cells and both IgG (p < 0.01) and sICAM-1 indices (p < 0.05), between QAlbumin and QsICAM-1 (p < 0.005) and between Qalbumin and the Expanded Disability Status Scale score (p = 0.05). There were no significant correlations in the Gd-negative group. These results suggest that sICAM-1 index can be a better marker of intrathecal sICAM-1 synthesis than CSF levels and provide additional insights, in vivo, into the blood-brain barrier mechanisms underlying MRI Gd-enhancement in clinically active RR MS.