Summary A retrospective analysis of thyroid scintiscanning at one hospital over a 3-year period revealed nineteen patients in whom a solitary, hyperfunctioning thyroid nodule was producing thyrotoxicosis. Most of the patients had clinical features referable to hyperthyroidism and in the majority a solitary nodule was palpable in the neck which was found to correlate well with the location of the ‘hot’ nodule on scanning. Cardiac failure was present in 21% and atrial fibrillation occurred in 37%. The 131I uptake by the thyroid was of little value in deciding whether or not the patient was toxic, but there was good agreement between serum protein bound iodine determinations and measurement of serum thyroxine and resin uptake tests. There appeared to be no relationship between the size of the nodule and its degree of activity.