The aim of the study was to determine the prevalence of thyroid autoimmune disorders in a cohort of untreated multiple sclerosis (MS) patients and compare it with a stratified sample of an adult population. We prospectively studied 93 untreated MS patients. The control group included 401 healthy subjects selected by stratified sampling in a non-iodine-deficient area. Antithyroid antibodies (ATA) (antibodies against peroxidase and thyroglobulin) were considered positive at titres > or =149 IU/ml. Antibodies were positive in 11 MS patients (11.8%; 95% CI 5.3-18.4%). This prevalence was five times higher (P = 0.0001) when compared with that in the control population. We found six cases with subclinical hypothyroidism (6.45%; 95% CI 11.4-1.5) in contrast to 2.24% in the control group. Comparing MS with positive and negative ATA, there was a non-significant, slightly higher frequency of low Expanded Disability Status Scale (EDSS) score in the ATA-positive group (81% vs. 73.2%). One year after start of interferon (IFN) treatment, only one patient developed subclinical thyroid dysfunction. MS patients have a higher prevalence of ATA compared with the general population. An initial ATA and thyroid-stimulating hormone (TSH) determination is recommended in all MS patients. A periodic assessment of thyroid function during IFN treatment only seems to be justified in those cases where positive ATA or dysfunction is present before treatment.