A parallel, randomized, double-blind trial is reported, in which 78 patients with chronic tension-type headaches were treated with l-5-hydroxytryptophan (5-HTP) ( n = 43) or placebo ( n = 35) for 8 weeks, after a washout period of 2 weeks and with a follow-up period of a further 2 weeks. Five patients dropped out (1 with placebo and 4 with 5-HTP), 1 was excluded due to noncompliance, and in 7 treatment was suspended due to adverse events (3 with placebo and 4 with 5-HTP), yielding a final number for evaluation of 65 patients. In comparison with the group treated with placebo, there was no statistically significant change in the number of days with headache or in headache intensity in the group treated with 5-HTP, 300 mg per day; however, there was a significant decrease in the consumption of analgesics. During the 2 weeks after treatment, there was a significant decrease in the number of days with headache. Subjective opinion during this latter period was also favorable to 5-HTP.