ABSTRACT– Twenty-eight elderly patients with dementia and in many cases also depression received daily treatment with either L-tryptophan (3g in the evening) or casein (control substance) for 1 monih each, using a crossover design and double-blind procedures. Each patient's mental condition and performance ability were rated, and biological variables (serum Iryptophan levels, platelet 5-HT up-take, platelet MAO activity, red blood cell thiamine, pyridoxine and riboflavin levels) were measured. Serum tryptophan levels were elevated by tryptophan treatment, but the treatment failed to have significant effects on the mental condition of the patients as a whole. Dividing the patients into two groups on the basis of the treatment during which they received their best behavioral rating failed to show significant relations to biological variables, except for differences between Vmax values for platelet 5-HT uptake. The findings indicate that long-term administration of tryptophan to gerontopsychiatric patients does not generally have beneficial effects.