Linomide is a potent immunomodulator and has been reported to prevent type 1 diabetes mellitus in non-obese diabetic (NOD) mice and to reduce the incidence of other autoimmune diseases in animal models. The mechanisms of action seem to involve antigen expression by down regulation of macrophage activity and to antagonise the activation of Th1 cells during the cellular immune response. With the purpose to investigate the effect of Linomide on the incidence of spontaneous autoimmune thyroiditis (AIT) in female NOD mice we administered Linomide in drinking water (100 mg/kg/day) to NOD mice from 5th to 19th week of age. The mice were sacrificed at the end of week 19. None of the mice developed diabetes during the study period. The incidence of thyroiditis was evaluated on paraffin HE-stained sections and graduated on a scale from 0 to 4. Thirty-two percent of 37 mice treated with Linomide developed thyroiditis compared to 45% of 22 controls (p=0.31, chi2 =1.00). Among the mice who developed thyroiditis no difference in the degree of thyroiditis was found. Therefore no beneficial effect of Linomide on the incidence of spontaneous AIT in NOD mice could be demonstrated.