Objective: The nature of ADHD, especially in adulthood, is not well-understood. Therefore, we explored subcomponents of attention in adult ADHD. Method: Twenty-three adults with ADHD were tested on neurocognitive and visual tracking performance both while on their regular prescription stimulant medication and while abstaining from the medication for 1 day. Pairwise comparisons to 46 two-for-one matched normal controls were made to detect medication-resistant effects of ADHD, and within-participant comparisons were made to detect medication-sensitive effects in patients. Results: Even when on medication, patients performed more poorly than controls on a spatial working memory task, and on visual tracking and simple reaction time tasks immediately following other attention-demanding tasks. Patients’ visual tracking performance degraded while off-medication in a manner consistent with reduced vigilance. Conclusion: There may be persistent cognitive impairments in adult ADHD despite medication. In addition, the benefit of stimulants seems reduced under cognitive fatigue.