We conducted a double-blind, placebo-controlled, crossover study of oral L-threonine at 6 g/day in patients with spinal spasticity. Muscle tone from selected leg muscles, measured by the Ashworth Scale, was the principal measure of spasticity and was evaluated before and at the end of each treatment period. A 10% reduction in Ashworth score was regarded as a positive response to a treatment. The results were analyzed sequentially, patients being classified as threonine-responders, placebo-responders or non-responders (those who responded to both treatments by either less or greater than 10%) and a level of significance of p = 0.05 was chosen. The trial concluded in favour of L-threonine after 33 patients. Side-effects were minimal. L-threonine has a modest but definite antispastic effect, and its possible role in modifying spinal glycinergic transmission is discussed.