Thyroid gland size was ultrasonographically determined in 35 pregnant women who live in an area with moderate iodine deficiency. Iodide salt was administered to group A (n = 17), whereas group B (n = 18) was used as a control. Each group was tested for thyroid-stimulating hormone serum levels, iodine excretion, and thyroid volume. In both groups thyroid-stimulating hormone levels were similar and did not change throughout pregnancy. The iodine excretion at the third trimester in the treated group was significantly (p less than 0.01) higher than that of group B (100.0 +/- 39.0 versus 50.0 +/- 37.0 micrograms iodine per 24 hours, respectively). Initially, thyroid volume did not differ between the two groups. At the end of pregnancy, no difference was found in thyroid size in group A, whereas in the untreated women it increased significantly (p less than 0.0001) with a mean increase of 1.6 +/- 0.6 ml (16.2% +/- 6.0%). These results show that the increased thyroid size in the control group was mainly a result of relative iodine deficiency and that iodoprophylaxis should be warranted even in areas with moderate iodine deficiency to prevent the increase in thyroid size and, probably, to avoid the risk of maternal and fetal hypothyroidism.