We evaluated the neurodevelopmental status and cognitive ability of 30 school-age children with Marfan syndrome. We found average intellectual (mean full scale IQ = 109.3) and gross motor development. Fifty percent had one or more neuropsychologic deficits: learning disability (LD) (13%), attention deficit disorder with or without hyperactivity (ADD +/- H) (17%), neuromaturational immaturity (NMI) (10%), and a performance IQ score different from their verbal score by more than 20 points (30%). Patients with a large verbal-performance discrepancy, with one exception, scored lower on tests of performance, and particularly low on two subtests, object assembly and coding, compared with the rest of the patients (P less than 0.01). The factor most highly correlated with verbal-performance discrepancy was the severity of joint hypermobility (P = 0.02), which suggests that the depressed performance score was due, in part, to motor incoordination. Children with Marfan syndrome may be prone to neuropsychologic problems; the role of routine screening remains to be tested. Those with hand-wrist hypermobility seem at particular risk for difficulties with tasks that involve writing and should receive careful evaluation in the classroom. Due to the high risk of aortic dilatation in these patients, if stimulant medications are considered for treatment of ADD +/- H, the risk of adverse sympathomimetic effect should be weighed against the potential benefit.