Due to high volatility and environmental mobility, radioactive isotopes of iodine pose a serious risk in the acute phases of a nuclear accident. The critical organ for iodine is the thyroid. A number of studies dealing with thyroid protection from exposure to radioiodine have shown that radioiodine uptake by the thyroid can be effectively blocked by administration of stable iodine, usually in the form of potassium iodide (KI) pills. However, unless perfectly timed, this protective action may be counterproductive. The International Atomic Energy Agency recommends potassium iodide prophylaxis in cases when an avertable thyroid dose by protective action exceeds 100 mGy. This paper reviews experiences and practices with potassium iodide in the thyroid protection. This kind of information should serve as the basis for discussion and decision making on KI prophylactic programmes in nuclear emergency situations in Croatia. If Croatia adopts such programme, it will still have to develop the most effective way of KI stockpiling and distribution or predistribution.