Aspects of attentional behaviour in 4--7-year-olds, as assessed by auditory and visual vigilance tasks, were related to prenatal exposure to cigarettes. Data on 76--79 children born to healthy, white, predominantly middle-class women were analyzed using multiple regression techniques controlling for potentially confounding variables including postnatal second hand smoke exposure. Cigarette smoking during pregnancy was related to an increased activity level of the children during the tasks and increased errors of commission in the auditory task and, to a lesser degree, in the visual task. After controlling for confounding factors, the relationship between prenatal smoking and errors of omission did not retain significance. The combination of results suggest that the deficits in attention may reflect impulsive responding and increased overall activity.