Recent studies have shown an association between anti-thyroid antibodies and autologous serum skin test (ASST) positive urticaria patients. However, a connection between thyroid and this reliable skin test for mast cell autoreactivity, ASST, has not been reported yet. We investigated ASST in patients with Hashimoto's thyroiditis (HT) without urticaria and compared the results with laboratory and sonographical findings of HT. 154 HT patients, 100 healthy volunteers without HT as a first control group and 46 patients with multinodular goitre but without autoimmune thyroid disease as a second control group underwent testing with ASST. ASST was applied to these groups according to two criteria, first as ASST(new): autologous serum red wheal response 1.5 mm bigger than negative control; second as ASST(old): serum red wheal response 5mmbigger than negative control accepted as positive. Free triiodothyronine (fT3), free thyroxine (fT4), thyroid-stimulating hormone (TSH), thyroid peroxidase antibody (anti-TPO) and thyroglobulin antibody (anti-Tg) levels were measured. ASST(old), ASST(new) scored positive in 51.3-60.4% of HT patients, with statistically significant differences. Thyroid volume grades were inversely proportional with ASST(old) and (new) positivity. Moderate (+) titers of anti-Tg in ASST(old) and (new) (+) cases were significantly higher than the same titers of anti-Tg in ASST(old) and (new) (-) cases. The prevalence of ASST positivity in HT patientswas not affected by the following factors: gender, age at screening, laboratory measurements of thyroid function tests, anti-TPO antibodies and thyroid ultrasound (US) echogenicity. Positivity of ASST in HT has shown that there is a skin mast cell autoreactivity in HT patients independent of autoreactive chronic urticaria (ACU).