The relationship of primary progressive multiple sclerosis (PPMS) to relapsing-remitting MS (RRMS) and secondary progressive MS (SPMS) remains unclear. Natural history data from a population-based cohort of patients with PPMS followed for approximately 25 years demonstrate remarkable similarities in the progressive phases of PPMS and SPMS. Immunogenetic and magnetic resonance imaging studies in large numbers of patients also fail to differentiate between the two MS categories. PPMS thus resembles SPMS without the relapses, although the two forms do differ with respect to sex ratio. An unfavourable outcome in PPMS in predicted by rapid early progression of disability and involvement of three or more systems. Natural history studies provide information on likely long-term outcomes and can be used in the design and interpretation of clinical trials in PPMS. The evidence that PPMS is distinct remains weak.