Korea's food culture includes the consumption of seaweed, which is abundant and has a high iodine content. Because it is customary to serve seaweed soup to new mothers, the consumption of seaweed increases dramatically when a woman is lactating. The present study was undertaken for the purpose of determining the iodine content in human milk of Korean lactating mothers according to dietary iodine intake. The iodine content of human milk and dietary iodine intake from 50 lactating mothers were analyzed at 2 to 5 days and at 4 weeks postpartum. The dietary iodine intake was assessed by the 24-hour recall method. The iodine content in human milk was analyzed by neutron activation analysis (NAA). The average daily iodine intake of lactating mothers was 2744 micrograms at 2 to 5 days postpartum, decreasing significantly to 1295 micrograms at 4 weeks postpartum. The major sources of iodine were seaweed (87%) and cows' milk (7%). The average iodine content in colostrum and mature milk was 2170 micrograms/l and 892 micrograms/l, respectively. There was a significant reduction in the levels of iodine in human milk depending on the lactation period. A significant correlation between maternal iodine intake and iodine content of human milk was observed (P < 0.0001). The frequency of seaweed soup intake in lactating mothers seems to be a modifying factor in the iodine intake level and the iodine content in human milk. The level of dietary iodine intake and the iodine content of breast milk of Korean lactating mothers is found to be much higher than in other countries.