Since the nineteen-sixties, L-tryptophan has been used to treat depression and sleep-disorders. It appears to be suitable for the the treatment of mild forms of depression and of special forms, such as depressive mood associated with the climacteric. In a subgroup of patients, L-tryptophan can help re-establish a physiological sleep pattern in patients with chronic sleep problems. In the case of dependence on hypnotics, L-tryptophan can ease withdrawal symptoms. Recently published studies have shown that acute L-tryptophan depletion can lead to impairment of sleep continuity, and, in patients with an appropriate predisposition, to brief reversible depressive moods. In 1989, oral preparations of L-tryptophan were reported to precipitate the eosinophilia-myalgia syndrome (EMS). However, the real culprit proved to be contaminations of the basic substance of L-tryptophan which are no longer present in measurable amounts in the preparations now available on the market. Today, improved purification and analytic methods ensure the harmlessness of L-tryptophan preparations. In comparison with synthetic antidepressants and hypnotics, L-tryptophan is characterized by a particular low level of side-effects.