Predicting clinical outcome is one of the major and most interesting issues in MS patients. If the global evolution of disability (usually chosen levels on the Kurtzke's Disability Status Scale) has been widely studied, much less is known about the progression of disability during the secondary progressive phase of the disease. It is usually said that there is a poorer prognosis once patients enter a progressive phase, whether from onset (like in primary progressive MS) or after an initial relapsing-remitting phase. The secondary progressive phase of multiple sclerosis (MS) is the one most often associated with the development of severe and irreversible disability. Despite this fact, there is a lot of information about the initial relapsing-remitting phase in the literature, but much less about the secondary progressive one. We review here information available from the literature and from the Lyon MS database about this secondary progressive phase.