Primary progressive multiple sclerosis (PPMS) is a rather unique form of the more common relapsing inflammatory demyelinative disease. The absence of attacks that typify relapsing forms of MS imposes special challenges for diagnosis, but also provides an opportunity to study the pathogenesis of the more progressive aspects of the disease process in isolation of confounding transient clinical events. In this review, recent advances in diagnostic approaches are considered in relationship to baseline data from a large multinational study designed to better characterize and treat this clinical phenotype. PPMS subjects with cerebral spinal fluid (CSF) findings consistent with intrathecal immunoglobulin production may have a more tissue destructive disease process than those whose CSF lacks evidence of a B-cell immunopathogenic disease component.