Although ultrasound during pregnancy is used extensively, there is little published on adverse fetal effects. We undertook a cohort study including men born in Sweden from 1973 to 1978 who enrolled for military service. We estimated relative risks for being born left-handed according to ultrasound exposure in fetal life using logistic regression analysis. Eligible for the study were 6,858 men born at a hospital that included ultrasound scanning in standard antenatal care (exposed) and 172,537 men born in hospitals without ultrasound scanning programs (unexposed). During the introduction phase (1973 to 1975) there was no difference in left-handedness between ultrasound exposed and unexposed (odds ratio = 1.03, 95% confidence interval (CI) = 0.91 to 1.17). When ultrasonography was offered more widely (1976 to 1978), the risk of left-handedness was higher among those exposed to ultrasound compared with those unexposed (odds ratio = 1.32, 95% CI = 1.16 to 1.51). We conclude that ultrasound exposure in fetal life increases the risk of left-handedness in men, suggesting that prenatal ultrasound affects the fetal brain.