Background: In the treatment of bipolar disorder in youths, often more than one medication should be prescribed. In the current study, we compared the efficacy and tolerability of the combination of lithium and quetiapine with lithium and risperidone in the treatment of manic or mixed episodes in children and adolescents. Methods: Thirty patients (aged 10-18 years) who were hospitalized for a manic or mixed episode were recruited from consecutive inpatient admissions to the Child and Adolescent Psychiatric Unit at Razi Psychiatric Hospital (University of Social Welfare and Rehabilitation Sciences, Tehran, Iran) from June 2012 to September. They were randomly treated with lithium (with the usual dose to achieve blood levels 0.8-1) and quetiapine (400-600 mg per day) or risperidone (0.5-6 mg per day). The primary outcome measure with respect to efficacy was the mean decrease in Young Mania Rating Scale (YMRS) score. Side effects were also assessed. The independent t test and two-factor repeated measure analysis of variance (ANOVA) was used for data analysis. P-value of less than 0.05 was considered statistically significant. Results: The reduction in YMRS scores was similar in both groups. The remission rate (YMRS <12) in the group treated with quetiapine was 80% and with risperidone was 66.6%; the difference was not significant. The most common side effect was sedation in both groups. Extrapyramidal side effects were observed only with risperidone. Both drugs caused increased levels of prolactin. Conclusion: Both protocols were effective. Quetiapine in combination with lithium in manic or mixed episodes of bipolar I disorder in children and adolescents was not superior to lithium and risperidone, but was associated with fewer complications.