Some features of cerebellar ataxia have been reported to regress partially with long-term administration of 5-hydroxytryptophan or levorotatory form of 5-hydroxytryptophan. To test this effect further, 30 patients with various inherited or acquired cerebellar ataxias underwent a randomized, double-blind trial of placebo, and the levorotatory form of 5-hydroxytryptophan taken orally for four months. The levorotatory form of 5-hydroxytryptophan significantly improved the ataxia score. It also significantly modified the time of standing upright, the spread of feet, the speed of walking, speaking, and writing. In five cases in which the levorotatory form of 5-hydroxytryptophan therapy was maintained for 12 months, the effect continued progressively.