SUMMARYSerial measurements of99Tcmuptake have been made in thirty thyrotoxic patients treated with carbimazole and L‐triiodothyronine. All patients were followed up for a minimum period of 1 year at which time eighteen patients were still in remission and twelve had relapsed.Four of the twenty‐one patients who had a normal99Tcmuptake at the time treatment was discontinued relapsed within 6 months. Only one of the nine patients whose uptake at the end of treatment was above normal achieved a remission. Changes in99Tcmuptake during the first 6 months of treatment were less informative and proved to be of limited value in predicting the outcome of treatment.In the whole group of patients99Tcmuptake fell during the first 6 months by about 40%, this change being highly significant. The decline in thyroid activity coincided with the time at which treatment was started. It is suggested that treatment with an antithyroid drug may modify the course of the dise