The study aimed to investigate whether the determination of chromosome aberrations in circulating blood lymphocytes could be useful to assess whole-body exposure from radioactive iodine released accidentally. Ten patients treated with two doses of 1850 MBq of 131I given 24 h apart for thyroid cancer were studied for chromosome aberrations (dicentrics) in blood samples taken before and at various times after exposure. The increase in the yield of aberrations caused by the exposure to iodine was small but statistically significant. Compared to published values for whole-body doses after such treatment, this increase appears to be somewhat smaller than expected from dose-effect relationships obtained for an acute exposure of lymphocytes in vitro or in vivo, a fact which could be explained by the low dose rate of the 131I exposure. Thus, in situations where a population was exposed as a result of the release of radioactive iodine, a determination of chromosome aberrations in blood lymphocytes would not appear to be very useful to determine exposure from iodine.