Although a relationship between attention-deficit/hyperactivity disorder (ADHD) and academic underachievement has been widely reported, the nature of this relationship has not been specified. The present investigation addresses this relationship directly by comparing 24 students (20 males and 4 females) with ADHD and 20 students (15 males and 5 females) with attention-deficit disorder without hyperactivity (ADD/noH) referred to a university-based diagnostic clinic for comprehensive neuropsychological assessment. The students ranged in age from 6 years 0 months to 12 years 10 months. Consistent with previous reports, this study found that math achievement test scores for students with ADD/noH were significantly lower than those for students with ADHD. These findings support previous research suggesting the ADD/noH may represent a distinct ADD subtype. It is hypothesized that inattention interferes with students' ability to master abstract symbol systems, especially in the acquisition of basic arithmetic skills in the primary grades.