The prevalence of academic underachievement or learning disabilities in children with autism or ADHD (two groups at increased risk for such problems) has been extensively researched, but little is known about overachievement in these groups. The frequency of overachievement across academic domains compared to nondiscrepant achievement and underachievement in children with autism versus ADHD was determined. WISC-III/-IV and WIAT/-II scores were analyzed in 164 children with autism and 499 children with ADHD (6-16 years of age, IQ [greater than or equal to] 80). Academic overachievement (achievement test standard scores [greater than or equal to] 1 SD above IQ) in word reading, reading comprehension, math, and written expression was rare in autism (14.6%, 9.1%, 5.5%, and 1.8%) and in ADHD (4.2%, 3.6%, 2.4%, and 0.8%, respectively). Academic underachievement (achievement test scores [greater than or equal to] 1 SD below IQ) was more common, with percentages of 12.8%, 17.7%, 15.2%, and 52.4% for autism and 27.9%, 21.4%, 24.0%, and 57.1% for ADHD. Overachievement was more common in autism than in ADHD. In contrast, underachievement was greater in ADHD than in autism, except that underachievement in written expression was similar and found in the majority of both groups.