The present clinic observation was conducted in the Rheumatology Clinic for the period September 1 2010-November 1 2010. The pain threshold scores were assessed in 21 patients using a Fisher dolorimeter and thus 3 men and 18 women aged 22-58 years were diagnosed with fibromyalgia (FM) (FM mean duration: 1-10 years). All patients were positive for IgG Epstein-Barr virus (EBV). In that way, only one group of patients was formed. In this group, there were prevailing patients with very high titer of IgG for EBV - in 16 of 21 patients the EBV IgG titers were 5 to 10 times above the normal control level. In 13 of 21 FM patients with high levels of EBV IgG titer (65%), a low concentration of adrenaline was observed (below the normal control level). Only in 6 of 21 patients, the concentration of noradrenaline in plasma was below the normal control level. In testing the concentration of serotonin in patients plasma, there were established low concentrations (up to 50 ng/l) in 8 patients and in 3 patients the concentrations were below the normal range, eg. 11 of 21 patients (50%) had low concentrations of serotonin. Therefore our results revealed a relationship between decreased pain threshold and serotonin. Perhaps even in patients with normal plasma concentrations of serotonin, disturbances in its transport through the blood-brain barrier (BBB) could be observed.