A comparative study of the outcome of surgical treatment for thyrotoxicosis was carried out in two countries with dissimilar dietary iodine levels. In the area with a high iodine level (Iceland) the prevalence of post-operative hypothyroidism was five times lower, but recurrent hyperthyroidism was five times higher, than in the area with lower iodine levels (northeast Scotland). The total morbidity reached comparable levels in the two samples. The prevalence of positive thyroid antibody tests and serum thyrotrophin levels was lower and the functional capacity of the thyroid remnant higher in the area with the higher dietary iodine intake. The study provides further evidence that there are important regional differences in the prevalence of factors known to influence the response to surgical treatment of thyrotoxicosis which should be taken into account when planning treatment services.