Serum and cerebrospinal fluid (CSF) soluble intercellular adhesion molecule-I (ICAM-I) levels were evaluated (ELISA) in 22 untreated and 13 corticosteroid-treated active relapsing remitting (RR) Multiple Sclerosis (MS), in 10 untreated and 10 corticosteroid-treated Guillain-Barré syndrome (GBS) and in 17 non-inflammatory neurological diseases (NIND). Twenty-eight clinically inactive RR MS were assayed for serum sICAM-I before and after 3 months treatment of 8 MIU rIFN beta-Ib taken s.c. every other day. High sICAM-I serum levels above the NIND values were found in untreated clinically active MS and in untreated GBS (P < 0.05) but not in the untreated clinically inactive MS group. The active MS group showed significantly (P = 0.0001) higher sICAM-I serum levels if compared to the inactive group. Corticosteroid-treated active MS and GBS patients showed lower (P < 0.05) serum sICAM-I levels than the corresponding untreated groups. Serum sICAM-I levels after 3 months of rIFN beta-Ib treatment (P < 0.0001, paired t-test) resulted increased compared to pretreatment values in MS. The mean values of CSF/serum sICAM-I:CSF/serum Albumin ratios (sICAM-I Index) in active untreated MS patients were higher compared to NIND (P < 0.005) and to corticosteroid-treated MS group (P = 0.01). sICAM Index values in GBS did not differ from those in NIND. The results seem to suggest potential roles for serum sICAM-I in downregulating the ongoing inflammatory response at the blood-brain barrier level and for CSF sICAM-I in the maintenance of a central nervous system local immune response.