Because iodine is added to cattle feed, milk and dairy products have become a major source of dietary iodine in Britain. The mean iodine concentrations of milk supplied to seven towns in England were measured each month throughout 1 year. Values were more than twice those recorded 20 years ago. There were large differences between the towns, with average annual concentrations ranging from 130 to 200 micrograms/l. Within the towns there were up to four-fold variations between the peak values in February/March and lowest values in June. The similarity between the monthly variations in milk iodine and seasonality in the onset of thyrotoxicosis suggests that iodine concentrations in milk and dairy products may now have reached harmful levels. Control of high winter levels in areas of Britain which were previously iodine deficient may now be necessary.