Objective: Despite the fact that autonomic nervous system (ANS) abnormalities are commonly found in adults and predict increased cardiovascular mortality, no studies have assessed ANS function in adults with attention-deficit/hyperactivity disorder (ADHD) taking stimulants. Method: This pilot study evaluated ANS function in adults with ADHD in comparison with an age- and gender-matched control group. Results: The authors found that 4% of the control group had some abnormalities in the ANS in comparison with 24% of the ADHD group. Conclusion: Because the control group had higher levels of exercise fitness, and the level of abnormalities in the ADHD group was comparable with that of the general population, the significance of these findings is unknown. In addition, we did not determine if ANS abnormalities were present in individuals with ADHD who were not on stimulant medications. Further research is warranted to determine if there is any association between ADHD and stimulant use and ANS abnormalities. (Contains 2 tables and 1 figure.)