This article summarizes case reports, population studies, and experimental studies from the literature concerning adverse effects of exposure to iodine from the mid-1880s to 1988. Exposure to excessive iodine through foods, dietary supplements, topical medications, and/or iodinated contrast media has resulted in thyroiditis, goiter, hypothyroidism, hyperthyroidism, sensitivity reactions, or acute responses for some individuals. Reports of maternal iodine exposure during pregnancy or lactation affecting newborn or nursing infants are cited. Susceptibility to excess iodine is discussed as well as the relationship between dose and response. It is concluded that some individuals can tolerate very high levels of iodine with no apparent side effects and that iodine intakes less than or equal to 1.000 mg/day are probably safe for the majority of the population, but may cause adverse effects in some individuals. Determination of maximum tolerable levels of iodine intake will require human experimental studies at levels between 0.150 and 1.000 mg/day for normal subjects, subjects with autonomous thyroid tissue, and iodine-sensitive subjects.