A girl aged 16 was diagnosed as suffering from thyrotoxicosis as age 14. She was treated with carbimazole until 3 months before her last admission. A week before admission she developed tremor, irritability, weakness and headache. Carbimazole treatment was resumed but a few days later she collapsed and on admission to Casualty was unconscious and in status epilepticus which was controlled with intravenous thiopentone. Convulsions returned, and there was peripheral circulatory collapse and tachycardia. There was a progressive rise in temperature and she died. At postmortem a large thymus and lymph nodes were found, together with uniform smooth swelling of the thyroid. Section of the thyroid showed marked proliferative activity with tall active epithelium and numerous lymph follicles containing active germ centres. Lymph nodes showed sinus reaction. The brain showed oedema and vascular congestion only.