Studies on iodine metabolism were performed in 17 patients with sporadic non-toxic goitre. A group of 11 normal subjects served as control. These investigations show that goitrous patients are at least iodine deficient: there is a low urinary excretion of stable and radioactive iodine with high thyroid uptake of radioiodine. Various rate constants of the iodide cycle in the body have been calculated. In particular, the concentration of stable iodide in the plasma as well as the pool of inorganic iodine are significantly reduced as compared with controls. The great avidity of the thyroid allowed an equal accumulation of stable iodine per unit of time in goitrous and non-goitrous subjects. Iodine deficiency undoubtly plays a role in the pathogenesis of sporadic non-toxic goitre in Belgium. The importance of lack of iodine is discussed in relation to other goitrogenic factors.