Serum thyroglobulin and thyrotropin as well as urinary iodine concentrations were measured in healthy, pregnant Swedish (n=27) and Sudanese (n=21) women and the results compared with those of healthy Swedish (n=14) and Sudanese (n=20) non-pregnant controls. The median thyroglobulin concentrations (and interquartile range) in the Swedish pregnant women for the three trimesters were 15.5 (8-24), 10.5 (7-19) and 18.0 (13-25) microg/L, respectively. The median third trimester concentration was higher than both the first and second trimester concentrations, respectively (p<0.0001, p<0.0001). Compared to the control group, the Swedish pregnant women had a significantly higher median thyroglobulin concentration in the third trimester (p<0.05). Among the Sudanese pregnant women, the median serum thyroglobulin concentrations (and interquartile range) were 27.5 (12-40), 25.0 (15-43) and 30.0 (15-67) microg/L during the first, second and third trimesters, respectively. There were no significant differences between these concentrations. Compared to the control group, the Sudanese pregnant women had a significantly higher median thyroglobulin in the third trimester (p<0.01). The Sudanese pregnant women also showed significantly higher median thyroglobulin concentrations than the Swedish pregnant women in all the three trimesters of pregnancy (p<0.05, p<0.001 and p<0.01, respectively). However, there were no significant differences between the two non-pregnant controls. Among the Swedish pregnant women, 40%, 23% and 30% of the subjects showed serum thyroglobulin concentrations above 20 microg/ L during the first, second and third trimesters of pregnancy, respectively. Corresponding figures for the Sudanese pregnant women were 55%, 61% and 64%, respectively. A significantly negative correlation was shown between serum thyroglobulin and urinary iodine concentrations during the second and third trimesters in the Swedish women (r= -0.8, p=0.01 and r= -0.5, p=0.03, respectively), and in the third trimester in the Sudanese women (r= -0.6, p=0.03). No such correlation was observed between thyrotropin and urinary iodine concentration in either the Swedish or the Sudanese pregnant women. It is concluded that serum thyroglobulin is a more sensitive indicator of iodine deficiency than serum thyrotropin during pregnancy.