In Thailand, iodine deficiency disorders (IDD) are endemic in 57 out of 75 provinces with an estimated 15 million people at risk of IDD.  A three pronged control program with iodized salt, iodized water and iodized oil capsules is being implemented. The water iodization program is both school based and household based. In the household, the residents are given iodine solution, two drops of which is to be added to 10 l of drinking water. In the schools, in addition to this method, an iodinator is used. This releases a fixed amount of iodine into the drinking water. This study examines the cost of the water iodization program in Thailand for the year 1996 in terms of cost per beneficiary, cost per µg iodine consumed daily and cost per goiter person years averted.  We used a discount rate of 5%. Field visit and interviews of health personnel from Ministry to village level were conducted to gather primary data. Review of existing papers and reports of the Department of Health, Government of Thailand was done for secondary data. The costs included the capital cost of equipments, initial training and the recurrent costs of potassium iodate, proportional salaries of personnel involved, monitoring and communication activities.  The cost per beneficiary of school based iodinator method (US$ 0.72) and school based drop method (US$ 0.64) were similar and much higher than the household based approach (US$ 0.12).  The cost per µg of iodine consumed daily was ten times higher in the school based approach (US$ 0.01) compared to the household approach (US$ 0.001).  The cost per goiter case averted for the whole strategy of water iodization was US$ 194.50.  Water iodization appears to be a low cost intervention. However, the need for behavioral modification raises the issue of long term sustainability.