We compared eight antithyroid peroxidase antibody assay kits in two centres, by use of panel sera from 269 patients: controls (n = 100), patients with autoimmune thyroid diseases (n = 77 Graves' disease, Hashimoto's thyroiditis), with non autoimmune thyroid diseases (n = 69 nodular goiter, differentiated thyroid carcinoma), and with autoimmune disease without thyroid pathology (n = 23 diabetic subjects, rheumatoid polyarthritis). On the controls sera we observed different distributions of values. The cut-off values of each kit was, in most cases, similar to the value noted in the manufacturer's instructions. In the clinical study, we observed few differences. The majority of assays demonstrated high diagnostic performance. Some false positive results and the non assessment of some sera by competitive immunoassay were observed.