OBJECTIVE: To review methods for evaluating iodine deficiency in pregnant women and young infants and to discuss factors to be considered in the interpretation of their results. DESIGN: Review of the literature regarding the various methods available for assessing iodine status.Setting: Population surveys and research studies. SUBJECTS: Pregnant women and young infants. RESULTS: Several factors to consider when assessing iodine status in pregnant women and young infants include: 1) the urinary iodine (UI) concentration (microg l-1) is not interchangeable with 24 h UI excretion (microg per 24 h); 2) the concentration of iodine in a spot or casual urine sample cannot be used to diagnose iodine deficiency in an individual; 3) a moderate fall in the concentration of serum free T4 during pregnancy is not a sign of maternal iodine deficiency; 4) an increase in the concentration of serum thyroglobulin (Tg) during pregnancy is not a sign of maternal iodine deficiency; 5) a higher concentration of TSH and Tg in cord blood than in maternal blood is not a sign of iodine deficiency in the mother or neonate; and 6) thyroid function in a full-term foetus, a neonate or a small child is not more sensitive to a mild iodine deficiency than in the mother. CONCLUSIONS: If the iodine status of pregnant women and small children is not to be misjudged, the above six factors need to be taken into account.