In a retrospective study 716 consecutive newly diagnosed and untreated hyperthyroid patients were examined in regard to the frequency of silent thyroiditis and postpartum thyroiditis. Six patients (0.8%) had possible silent thyroiditis (24-hour radioiodine uptake less than or equal to 5% and hyperthyroidism without anterior neck pain). None occurred within one year postpartum. We conclude that silent thyroiditis is a rare cause of hyperthyroidism in our thyroid clinic. The fact that none occurred postpartum suggests that postpartum thyroiditis is oligosymptomatic and that screening programmes are needed if one wants to diagnose the earliest phases of this condition.