5-HT precursors are used in depressions on the basis of the 5-HT hypothesis--a hypothesis which postulates that a cerebral 5-HT deficiency can play a role in the pathogenesis of depressions. This article presents a survey of the results obtained by this therapeutic strategy. There are strong indications that 5-HTP is of therapeutic value, particularly in the 5-HT-deficient subgroup of vital depressions. In the same subgroup, one controlled study has so far also shown a prophylatic 5-HTP effect. The results of tryptophan studies are less unequivocal, possibly due to pharmacokinetic factors. Another possible explanation might be that, unlike 5-HTP research, tryptophan studies have so far disregarded the patient's serotonergic status. 5-HT research in depressions has also yielded the concept of the biochemical classifiability of depressions. This may become an important supplement to the conventional criteria of classification of depressions: symptomatology, etiology, and course.