Objective: To determine the effects of long-term methylphenidate treatment on symptom severity and social adjustment in adult ADHD. Method: Adults (n = 116) meeting operational diagnostic criteria for ADHD (the "Utah Criteria") entered a randomized double-blind crossover trial of methylphenidate and placebo. Participants who improved on immediate-release methylphenidate entered a 12-month, open-label trial. Outcomes were assessed using the Wender-Reimherr Adult Attention Deficit Disorder Scale (WRAADDS), Clinical Global Impression-Improvement (CGI-I), global assessment of functioning (GAF), and the Weissman Social Adjustment Scale (WSAS). Results: In the double-blind trial more patients improved (50% reduction of symptoms) receiving methylphenidate (74%) than placebo (21%, p = 0.001). During the open-label trial, symptom severity decreased 80% from baseline, and the WSAS decreased greater than 50% in all subscales. The average GAF improved significantly (p less than 0.0001). Conclusion: ADHD adults, who responded to methylphenidate in a short-term, placebo-controlled trial, responded to long-term treatment with marked improvements in ADHD symptoms and psychosocial functioning. (Contains 4 tables.)