Food composition tables were used to estimate the iodine intake of 105 men and 112 women living in Cambridge who completed 7-day semi-weighed records of food consumption. Intakes in men ranged from 195 micrograms per day in summer to 306 micrograms in winter, and in women from 126 micrograms to 236 micrograms. Seasonal variations in intake were mainly due to seasonal differences in the iodine content of milk which varies directly in relation to farming practices. The authors speculate that a spring-summer peak in thyrotoxicosis incidence in Britain may be causally related to the high milk iodine levels in winter.