In clinical samples, juvenile bipolar disorder (JBPD) is frequently accompanied by co-morbid attention-deficit/hyperactivity disorder (ADHD). Clinical trials assessing combined psychopharmacological interventions in this population are scarce, and methylphenidate (MPH) may worsen manic symptoms. We conducted a randomized crossover trial with MPH and placebo (2 weeks each) combined with aripiprazole in children and adolescents (n = 16; 8-17 years old) with JBPD and ADHD who had a significant response in manic symptoms with aripiprazole but still presented clinically significant symptoms of ADHD. ADHD, manic, and depressive symptoms were assessed by means of standard scales. Fourteen out of the 16 subjects completed the trial. No significant differences between the effects of methylphenidate and placebo were detected in ADHD (F(1, 43.22) = 0.00; p = 0.97) or manic (F(1, 40.19) = 0.93; p = 0.34) symptoms. Significant improvement in depressive symptoms was observed in the MPH group (F(1,19.03) = 7.75; p = 0.01) according to a secondary self-reported outcome measure. One patient using aripiprazole and MPH discontinued the trial due to the onset of a severe mixed episode. No other significant adverse events were observed. Although MPH did not worsen manic symptoms, it was not more effective than placebo in improving ADHD symptoms in children and adolescents with JBPD co-morbid with ADHD stabilized with aripiprazole. Further investigations are warranted. This study is registered at www.clinicaltrials.gov under the identifier NCT00305370.