OBJECTIVE: To determine the efficacy and safety of clonidine, used alone or in combination with methylphenidate, in treating attention-deficit/ hyperactivity disorder (ADHD). METHOD: A 16-week, randomized, double-blind, placebo-controlled clinical trial was conducted in 122 children, ages 7 to 12, with any subtype of ADHD, randomly assigned to clonidine, methylphenidate, clonidine in combination with methylphenidate, or placebo according to a 2 × 2 factorial design. In two successive 4-week titration periods, clonidine (or matching placebo) and added methylphenidate (or matching placebo) were adjusted to optimal doses and then continued for 8 weeks. The primary efficacy outcome was changed from baseline to week 16 on the Conners Teachers Abbreviated Symptom Questionnaire. Secondary outcomes included the Conners Abbreviated Symptom Questionnaire for Parents and the Children's Global Assessment Scale. RESULTS: On the Conners Teachers Abbreviated Symptom Questionnaire, clonidine was not found to improve ADHD symptoms, whereas subjects treated with methylphenidate showed significant improvement compared to those not treated with methylphenidate. Subjects treated with clonidine had greater improvements on the Conners Abbreviated Symptom Questionnaire for Parents and Children's Global Assessment Scale, but also a higher rate of sedation compared with subjects not treated with clonidine. CONCLUSIONS: Based on the Conners Teachers Abbreviated Symptom Questionnaire, methylphenidate offers the best combination of efficacy and tolerability for ADHD. Clonidine was well tolerated despite the frequency of sedation and did offer some benefit. Copyright 2008 © American Academy of Child and Adolescent Psychiatry.