The separate correlations among CSF and clinical parameters (mononuclear cell count, assay of intrathecal IgG synthesis, assessment of blood-brain barrier permselectivity to albumin and alpha 2-macroglobulin, detection of oligoclonal intrathecal IgG isoelectric spectro-type; age, disease duration, disability, number of bouts) were investigated by multiple linear regression in 100 multiple sclerosis patients subdivided according to the course and the phase of the disease. Results indicate that the complexity of the oligoclonal CSF IgG isoelectric spectrotype increases, but the overall amount of intrathecal IgG synthesis decreases with increasing number of bouts or disease duration in relapsing remitting and in progressive courses respectively. The CSF mononuclear pleocytosis was low in progressive forms, and it appeared to be linked to the intrathecal IgG synthesis changes. The IgG synthesis rate was proportional to CSF pleocytosis, but a negative correlation between pleocytosis and complexity of CSF IgG spectrotype was found. About 40% of patients had a blood-brain barrier damage. The increased permeability to albumin was related to the number of previous bouts, whereas the CSF pleocytosis seemed to be accompanied by a loss of barrier selectivity to large serum molecules. High CSF IgG index, high mononuclear cell count and low number of prominent oligoclonal CSF IgG fractions characterize the less active or short-lasting disease. Low CSF IgG index with numerous, faint, abnormal IgG CSF bands, low mononuclear cell number and increased barrier permeability to albumin are associated with long-lasting or steady progressive disease.