Biological dosimetry was applied to 30 patients with differentiated thyroid carcinoma who were treated with 131I (3.7 GBq) to ablate thyroid remnants after surgery or in case of metastases. METHODS Chromosomal aberrations were scored in peripheral blood samples obtained before and 4 days after the first administration of 3.7 GBq 131I according to two methods: conventional cytogenetics and chromosome 4 painting. This generated a dosimetric index that reflects the dose to the bone marrow. RESULTS Results of both techniques were in close agreement. The mean dosimetric index at Day 4 was 0.54 Gy (95% CI: 0.45-0.62 Gy) by conventional cytogenetics and 0.48 Gy (95% CI: 0.42-0.61 Gy) by chromosome 4 painting. This dose is 2-4 times higher than that derived from the MIRD estimates. Since blood was drawn at Day 4, this technique underestimates the dose by at least a third. The high dose estimate may be related to the hypothyroid status of these patients at the time of 131I administration, a condition which decreases renal clearance of 131I and thus increases whole-body irradiation. This dose estimate was closely related to whole-body retention of 131I at Day 4. CONCLUSIONS These data should be used to estimate the risk due to 131I exposure.