The relationship between right hemisphere dysfunction and attention-deficit/hyperactivity disorder (ADHD) remains controversial. We administered a random letter cancellation test to 58 carefully selected adult patients meeting DSM-IV criteria for ADHD and 29 age- and education-matched controls. Patients with ADHD had a higher mean omission rate on the left side than the controls, and a greater percentage of ADHD patients than controls made more omissions on the left than on the right (L > R errors). ADHD patients who made L > R errors had lower performance IQ scores than ADHD patients who did not make L > R errors. However, ADHD patients who made L > R errors did not differ from ADHD patients who did not make L > R errors in ADHD subtype, medication response, or neuropsychological measures of attention, executive function, verbal memory, nonverbal memory, or academic achievement. Patients without a family history of ADHD were more likely to make L > R errors than patients with a family history of ADHD. This study provides support for the concept of right hemisphere dysfunction in a subset of patients with ADHD. However, ADHD patients who make L > R errors do not appear to represent a distinct clinical subgroup in terms of medication response, ADHD subtype, or neuropsychological test performance.