OBJECTIVE To determine whether exposure to low doses of ionising radiation in infancy affects cognitive function in adulthood.   
 DESIGN Population based cohort study.   
 SETTING Sweden.   
 PARTICIPANTS 3094 men who had received radiation for cutaneous haemangioma before age 18 months during 1930-59.   
 MAIN OUTCOME MEASURES Radiation dose to frontal and posterior parts of the brain, and association between dose and intellectual capacity at age 18 or 19 years based on cognitive tests (learning ability, logical reasoning, spatial recognition) and high school attendance.   
 RESULTS The proportion of boys who attended high school decreased with increasing doses of radiation to both the frontal and the posterior parts of the brain from about 32% among those not exposed to around 17% in those who received > 250 mGy. For the frontal dose, the multivariate odds ratio was 0.47 (95% confidence interval 0.26 to 0.85, P for trend 0.0003) and for the posterior dose it was 0.59 (0.23 to 1.47, 0.0005). A negative dose-response relation was also evident for the three cognitive tests for learning ability and logical reasoning but not for the test of spatial recognition.   
 CONCLUSIONS Low doses of ionising radiation to the brain in infancy influence cognitive abilities in adulthood.
True