Male vulnerability to neurodevelopmental disorders remains controversial. For one disorder, reading disability, this sex bias has been interpreted as an artifact of referral bias. We investigated sex differences for the incidence of reading disability within a large prospective sample of White (N 5 16,910) and Black (N 5 15,313) children derived from the National Collaborative Perinatal Project (NCPP). Children were classified as having either moderate or severe reading disability when they had reading scores lower than 1.5 or 2.0 standard errors of prediction, respectively, given their age and intelligence. Reading disability was about twice as common in boys than girls ( p , .001), irrespective of race, severity of disability, or exclusion of children with attentional disturbances or high activity levels. We conclude that there is a clear sex bias toward males for the incidence of reading disabilities. (JINS, 2000, 6, 433-442.)