The aim of the study was to evaluate the kinetic parameters of a specific serotonin transporter (SERT) and serotonin uptake in a mentally healthy subset of patients with fibromyalgia. Platelets were obtained from 40 patients and 38 healthy controls. SERT expression and functionality were evaluated through the measurement of [3H]paroxetine binding and the [3H]serotonin uptake itself. The values of maximal membrane binding capacity (Bmax) were statistically lower in the patients than in the healthy volunteers, whereas the dissociation constant (Kd) did not show any statistically significant variations. Moreover, a decrease in the maximal uptake rate of SERT (Vmax) was demonstrated in the platelets of patients, whereas the Michaelis constant (Km) did not show any statistically significant variations. Symptom severity score (tiredness, tender points index and Fibromyalgia Impact Questionnaire) were negatively correlated with Bmax and with Vmax, and positively correlated with Km. A change in SERT seems to occur in fibromyalgic patients, and it seems to be related to the severity of fibromyalgic symptoms.