Despite the increasing awareness of attention-deficit/hyperactivity disorder (ADHD) in adults, there are a limited number of controlled pharmacologic studies of this disorder. Because the stimulant medication magnesium pemoline (Cylert, Abbott Laboratories, Abbott Park, IL) has been found effective in treating ADHD in pediatric groups, we tested its efficacy in adults with ADHD using higher daily doses than those previously studied. We conducted a 10-week, double-blind, placebo-controlled, crossover design study of pemoline at a target daily dose of 3 mg/kg per day in 35 adult patients with DSM-III-R and -IV ADHD. We used standardized structured psychiatric instruments for diagnosis. To measure improvement, we used separate assessments of ADHD, depressive, and anxiety symptoms at baseline and at each biweekly visit. ADHD outcome was determined using the ADHD symptom checklist and Clinical Global Impression scales of Severity and Improvement. Of the 35 adults with ADHD who were randomized in the trial, 27 (77%) completed the protocol. Treatment with pemoline in the final week of the 4-week active phase was best tolerated at doses substantially lower than the target dose of 3 mg/kg per day (mean dose, 2.2 mg/kg per day; mean+/-SD, 148+/-95 mg). Pemoline was significantly better at reducing ADHD symptoms compared with placebo (z = 2.4,p < 0.02). Using a predefined 30% reduction in symptoms as an indication of improvement, 50% of pemoline-treated subjects and 17% of subjects in the placebo group were considered positive responders (chi2 = 7.1, p = 0.008). These results indicate that pemoline is moderately effective in the treatment of ADHD in adults. Although robust doses were targeted, most adults preferred more moderate dosing (120-160 mg/day). Given the limited efficacy, tolerability, and concerns of hepatic dysfunction, pemoline should be considered as second-line medication for treating ADHD in adults.