In order to examine whether iodine supplements may have caused a global decline in sperm concentrations during the past several decades, the synchronicity of the decline in mean sperm counts and the introduction of io-dine supplements was analyzed statistically. A positive correlation between the incidence of thyroid disease and sperm counts has been detected in Europe. In addition, it has been shown that sperm counts began falling around 1965 in the United States, 40years after iodine supplements were introduced. Mean sperm counts before and after 1965 were 111 10 sup(6)/ml and 70 10 sup(6)/ml, respec-tively, in calculations weighted by the number of subjects included in each in-dividual publication. The timing of the declines coincided with the introduc-tion of iodine supplements in the United States, France, and the United King-dom. The implications are that the global decline in sperm concentrations may be caused by iodine intake. Geographical variation in the types of sperm count decline also appears to be present.