Six patients with myoclonus of varying cause were treated with L-5-hydroxytryptophan (L-5-HTP) and carbidopa. While spontaneous myoclonus decreased in three of the patients and action myoclonus in four, only two patients had marked functional improvement. Side effects included gastrointestinal and affective disturbances. L-5-HTP therapy caused a diminished frequency of paroxysmal discharges in the electroencephalograms of three patients which did not always correlate with clinical improvement. Lumbar cerebrospinal fluid 5-hydroxyindoleacetic acid (5-HIAA) concentration after probenecid was decreased in all patients prior to therapy, but this reduction did not predict treatment response. Urinary excretion patterns for 5-HTP, serotonin, and 5-HIAA during treatment were similar in responders and nonresponders. It is concluded that while some patients with myoclonus do benefit from L-5-HTP therapy, biochemical and electrophysiological tests are not useful predictors of treatment response, and the high incidence of side effects limits the usefulness of this therapy.