The increasing use of fluoride for prevention of dental caries poses the problem as to whether this halogen has antagonistic properties towards iodine, whereby it could hamper the success of iodine prophylaxis of endemic goitre. Review of the literature shows that some authors have found an inhibition by fluoride of various steps of thyroid hormone biosynthesis in animal experiments. By and large, the inhibition was only slight and it was elicited only with fluoride doses greatly in excess of those recommended for caries prevention. The inhibition was not consistently present and other authors could not confirm it in comparable experiments. There is no convincing evidence that fluoride produces true goitres with epithelial hyperplasia in experimental animals. There are some reports based on casual observations that fluoride is goitrogenic in man. On the other hand, several good studies with adequate exposed and control populations failed to detect any goitrogenic effect of fluoride in man. It is noteworthy in particular that fluoride does not potentiate the consequences of iodine deficiency in populations with a borderline or low iodine intake. Published data fail to support the view that fluoride, in doses recommended for caries prevention, adversely affects the thyroid.