This paper reports on a development of the Rivermead Motor Assessment Gross Function scale, the Rivermead Mobility Index (RMI), a new measure of mobility disability which concentrates on body mobility. An early development included a second scale concentrating on elective mobility, but the results showed this to be unreliable. The RMI comprises a series of 14 questions and one direct observation, and covers a range of activities from turning over in bed to running. Its inter-observer reliability was tested on two groups of patients (n = 23 and 20 respectively) and it is reliable to a limit of 2 points (out of 15). Its validity as a measure of mobility after head injury and stroke was tested by concurrent measurement of mobility using gait speed and endurance, and by standing balance. The RMI does form a scale. It is short, simple, and clinically relevant, and can be used in hospital or at home.