We determined the frequency of thyroid autoantibodies in fibromyalgia (FM) patients and the relationship between FM symptoms and these antibodies. Euthyroid 128 FM patients, 64 rheumatoid arthritis (RA) patients, and 64 healthy control subjects were included in the study. The sociodemographic features and the clinical features of FM patients were determined. By using a visual analog scale, patients were questioned about the severity of FM-related symptoms. All patients were administered with Duke-Anxiety Depression (Duke-AD) scale, the physical function items of the fibromyalgia impact questionnaire scale. Thyroid autoimmunity was defined as the presence of detectable antithyroglobulin (TgAb) and/or antithyroid peroxidase (TPOAb) antibodies by the immunometric methods. Patients with a connective tissue disorder, hypo- or hyperthyroidism, and patients who had psychiatric treatment within the last 6 months were not included into the study. The frequencies of thyroid autoimmunity in FM (34.4%) and RA (29.7%) patients were significantly higher than controls (18.8%) (p<0.05). Twenty-six (20.3%) FM patients had positive TgAb and 31 (24.2%) had positive TPOAb. When patients with thyroid autoimmunity were compared to others, it was seen that the mean age, the percentage of postmenopausal patients, the frequency of dryness of the mouth, and the percentage of patients with a previous psychiatric treatment were higher in this group (p<0.05). FM patients had thyroid autoimmunity similar to the frequency in RA and higher than controls. Age and postmenopausal status seemed to be associated with thyroid autoimmunity in FM patients. The presence of thyroid autoimmunity had no relationship with the depression scores of FM patients.