The efficacy and tolerability of 5-hydroxy-L-tryptophan (5-HTP) were studied in an open 90-day study in 50 patients affected by primary fibromyalgia syndrome. When all the clinical variables studied throughout the trial (number of tender points, anxiety, pain intensity, quality of sleep, fatigue) were compared with baseline results, they all showed a significant improvement (P less than 0.001). The overall evaluation of the patient condition assessed by the patient and the investigator indicated a 'good' or 'fair' clinical improvement in nearly 50% of the patients during the treatment period. A total of 15 (30%) patients reported side-effects but only one patient was withdrawn from the treatment for this reason. No abnormality in the laboratory evaluation was observed. It is concluded that 5-HTP is effective in improving the symptoms of primary fibromyalgia syndrome and that it maintains its efficacy throughout the 90-day period of treatment.