Serum triiodothyronine (T3) and thyroxine (T4) were measured in thirteen unselected hyperthyroid patients before and at frequent intervals after iodine-131 treatment. There was an abrupt rise in T3 from a pretreatment mean of 250 ng/100 ml to a maximum of 365 ng/100 ml at 24 hours. Total T4 rose from a mean of 18.7 mug/100 ml to a maximum of 22.3 mug/100 ml at 24-48 hours. These findings may explain why an occasional patient develops acute thyroid crisis after radioiodine treatment.