The Neurophysiological Index (NI) consists of a mathematical derivation of three standardised neurophysiological measurements. Since these measurements are part of routine practice in any clinical neurophysiology laboratory, calculation of the NI is within the capability of any laboratory. The NI is derived from the CMAP, the DML and the F-wave frequency (CMAP amplitude/DML) x F frequency %), representing aspects of the effects of denervation and reinnervation, of degeneration of the terminal part of the motor axons, and of the excitability of anterior horn cells. We have shown that this simple index is reproducible in consecutive studies of normal subjects and of patients with ALS (intra-rater reliability), and is sensitive to change. In ALS, the NI differentiates rapidly and slowly progressive disease at least as sensitively as other measures in common use, including the ALS-FRS. We propose that the NI could be used as a sensitive measure of change during the course of ALS and its treatment. In combination with relevant measures of clinical benefit, such as ALS-FRS and a QoL measure, this could simplify trial design and allow more rapid determination of the efficacy of putative new therapies in clinical trials.