Thyroid uptake of 131I was measured in 130 volunteers following the nuclear power plant accident at Chernobyl in April 1986. Ninety of these volunteers had been travelling in different parts of eastern Europe at the time of or immediately after the accident while 40 persons were permanently in Sweden. Also, 28 additional healthy volunteers, living in Sweden, were chosen for a long-term follow-up of the time-course of 134Cs and 137Cs whole body uptake. The highest levels of 131I were found in persons having visited Poland (mean value 3.27 kBq +/- 3.68 SD, extrapolated to April 27) while persons that had stayed in other parts of eastern or northern Europe showed significantly lower levels (p less than 0.01). The whole body burdens of cesium radionuclides were barely detectable immediately after the accident but increased gradually throughout the observed period. After five months nine farmers from a high fallout area in central Sweden had reached mean values of 4.20 kBq (+/- 3.34 SD) of 134Cs and 137Cs while six nonfarmers from the Stockholm area showed significantly lower levels, 0.64 kBq (+/- 0.24 SD, p less than 0.05). The radiation doses from the observed amounts of iodine and cesium isotopes reported in this study reflect only a marginal addition to the already existing dose from the natural environmental background radiation.