L-Tryptophan (L-TRP) and the competing amino acids valine, leucine and tyrosine were determined in 120 non-depressive and in 107 depressive patients, subdivided according to DSM-III as minor depression (309.00, 300.40, 296.82), major depression without melancholia (296.X2), with melancholia (296.X3) and with psychotic features (296.X4). The ratio of L-TRP to the sum of the competing amino acids valine and leucine (CAA) was calculated and the Hamilton Depression Rating Scale (HDRS) score was determined. Neither in L-TRP nor in L-TRP/CAA there were significant differences between the depressive and the non-depressive population. Within the full depressive sample, patients with a major depression show significantly lower values for L-TRP and L-TRP/CAA ratio than those with minor depression, but the sensitivity is low. L-TRP and L-TRP/CAA show a significant (p < 0.001) correlation with the HDRS score. The decreased values of L-TRP and L-TRP/CAA ratio are characteristic of the severity of the depression and, accordingly, for the most severe subtypes of depression, viz. major depression with melancholia and with psychotic features. The decrease of L-TRP in these subcategories of depression may be regarded as a strain phenomenon introduced by the severity of the disease.