In this study we examined the technique of measuring thyroid-stimulating hormone (TSH) on filter paper blood samples for use in evaluating the iodine nutrition status of newborns and adults living in iodine-deficient areas. Filter paper blood samples were obtained between the 5th and 7th day after birth from 103, 43 and 103 term newborns living in Khartoum (mild iodine deficiency), Kosti (moderate iodine deficiency) and Darfur (severe iodine deficiency), respectively. TSH was measured with a commercial assay and the levels were compared with those obtained with the same method in 1147 samples from term Swedish newborns, obtained on the 3rd to the 5th day of life. The mean (95% confidence interval) TSH levels of the three Sudanese groups and the Swedish group were 7.1 (4.8-9.4), 8.3 (6.6-10.1), 11.9 (0.9-22.9) and 4.51 (3.8-5.3) mU l-1, respectively. The mean TSH for all three Sudanese groups was higher than the Swedish mean (p < 0.001). TSH levels determined in filter paper blood samples from adults living in an iodine-deficient area showed a correlation (p < 0.001; r = 0.55) to levels in corresponding serum samples (range 0.52-14.1 mU l-1, median 3.4 mU l-1). No significant correlation was found, however, between blood spot levels and serum levels within the reference range (< 5 mU l-1). A modification of the commercial procedure consisting in using three instead of two monoclonal antibodies did not sufficiently improve the assay for measurements of TSH within the reference range. Thus, there is a need to develop the filter paper technique further to make it a useful test for monitoring iodine deficiency in populations expected to have TSH levels close to the reference range. In the examination of neonates, however, with their birth-induced surge of TSH, the current assay promises to be a convenient tool for discovering iodine deficiency within a community.