In an open study 25 depressed patients were treated with L-5-hydroxytryptophan (L-5-HTP) either alone or in combination with a peripheral decarboxylase inhibitor. The therapeutic efficacy of L-5-HTP was considered as equal to that of traditional antidepressants. There was no difference in efficacy between the two treatments. Best results were obtained in patients with an anxious-agitated depressive syndrome and in patients with an endogenous depression if the illness had been acute. The onset of action was rapid (within 3 or 5 days). Gastrointestinal side effects proved to be dose-dependent and occurred more frequently in patients receiving L-5-HTP alone, whereas psychopathological side effects (especially acute anxiety states) have mainly been reported in patients receiving L-5-HTP in combination with a peripheral decarboxylase inhibitor.