Thirty-nine subjects with chronic insomnia were treated with L-tryptophan (L-TRP) in a double-blind, cross-over study. Instead of a placebo, a very low dose of 0.04 g L-TRP was used. The subjects suffered from a sleeping disorder classified as "psychophysiological, persistent". In the subgroup taking the full L-TRP (2 g) dose first, there was a significant difference between the treatment period with the full L-TRP dose and the ineffective dose (placebo). If the placebo was given first, however, there was no significant difference between the two treatment periods. It is suggested that psychological factors are responsible for the diverging results in the two subgroups of patients. On the basis of subjective ratings, it appears that L-TRP is effective in promoting sleep in cases of chronic insomnia.