We compared cognitive functioning, academic ability, and the predictors of academic underachievement in children with neurofibromatosis type 1 (NF1) (n = 132), children with NF1 and comorbid attention deficit hyperactivity disorder (NF1 + ADHD) (n = 60), and unaffected controls (n = 52). Results indicate the presence of ADHD burdens some aspects of cognitive functioning and learning in NF1. Inattention and executive dysfunction are general characteristics of the NF1 cognitive phenotype and significantly undermine academic achievement across children with NF1.