Radiation from the Chernobyl nuclear power plant meltdown greatly affected several Norwegian counties. The cognitive consequences of in utero exposure to radiation from the Chernobyl accident have been intensely debated. This study examines the cognitive outcomes for those Norwegians who were exposed as fetuses to the fallout from Chernobyl. The participants, 84 adolescents who were exposed in utero to radiation from the most contaminated areas in Norway and 94 adolescents from areas not contaminated by the radiation, were tested on verbal and nonverbal IQ. Two data analyses were conducted. First, using a control-group design, the IQ scores of exposed and unexposed adolescents were compared. Second, in a timing-of-exposure design, those exposed during the most sensitive period were contrasted with those exposed later in gestation. Adolescents exposed to low-dose ionizing radiation in utero scored significantly lower in full-scale IQ than unexposed adolescents. The difference was restricted to verbal IQ and was not evident for nonverbal IQ. The effect was not observed in exposed adolescents who had passed the most sensitive gestational period prior to the accident and thus were exposed to the radiation from Chernobyl exclusively after gestational week 16. These participants performed as well as the controls. Although the results should be interpreted cautiously due to the study's nonrandomized design, the data add new and important support to the hypothesis that the Chernobyl accident may have had a subtle effect on the cognitive functioning of those exposed to low-dose ionizing radiation in utero during the most sensitive gestational period.