SummaryA female patient with multinodular goiter was treated with methimazole (MMI) for thyrotoxicosis. After 3 weeks, she developped sore throat, pruritus, arthralgias, facial oedema and fever. Biological data were suggesting a scrum sickness. To our knowledge, this is the first published case report of MMI-induced serum sickness.Acta Clin Belg. 38, 1. 68–9The thioamide antithyroid drugs, propylthiouracil (PTU) and methylmercaptoimidazole (methimazole or MMI), share several side-effects, including skin rashes, neutropenia, agranulocytosis, jaundice and connective tissue disease-like syndromes (1). Vasculitis and serum sickness were also reported with PTU (1, 2). We describe here a case of serum sickness following the administration of MMI.