The ability to predict the development of disability in a disease as variable as multiple sclerosis is one of the major challenges facing researchers and clinicians alike. It focuses researchers on the mechanisms underlying irreversible deficit, while challenging clinicians to define criteria for the use of partially effective treatments with a not inconsequential side effect profile and cost. The concept of benign multiple sclerosis is of particular relevance to both these issues. However, although, as has been pointed out by Hawkins and McDonnell (pp 148–52, this issue) it has been recognised for over 40 years, the precise definition is extremely vague and would probably best be met by an index of progression rather than an arbitrary disease duration/Kurtze expanded disability …