The psychostimulants are the most widely used treatment modality for attention deficit/hyperactivity disorder (ADHD). Although positive responses to psychostimulants have been demonstrated, 46% of adults with ADHD, on average, are nonresponsive or cannot tolerate the medications because of adverse effects. We reviewed the charts of 43 students treated with pemoline and studied parameters of nonresponse. Sixteen (37%) met the study definition of nonresponse. Of these 5 (12%) were symptom nonresponders and 11 (25%) were nonresponders because of adverse effects. We found a significant relationship between age and nonresponse resulting from adverse effects, yet no other factors predicted nonresponse to pemoline. Clinicians should consider using smaller starting doses of pemoline in adults to minimize adverse effects.