Objectives: The aim of this study was to evaluate the clinical characteristics and diagnostic stability of children and adolescents with bipolar disorder. Method: In this prospective study, 257 subjects with bipolar disorder who were consecutively admitted to Roozbeh Hospital (Tehran, Iran) were enrolled. Demographic characteristics, diagnosis, treatment, comorbid disorders and mood and psychotic symptoms were extracted from the patients’ admission files and the information questionnaire, which had been filled in the admission time by the patients. In the further assessment, diagnostic evaluation was done using the Schedule for Affective Disorders and Schizophrenia for School-aged  children present and lifetime version (K-SADS-PL), for the patients under 18, and the Schedule for Affective Disorders and Schizophrenia (SADS) for the patients older than 18. Severity of the symptoms in the acute phase of mood disorder was evaluated by the Young Mania Rating Scale (Y-MRS) in manic phase and Beck Depression Inventory (BDI) for patients older than 16 or Children Depression Inventory (CDI) for the patients under 16 in depressive phase. Also, current and past levels of functioning were evaluated by the Global Assessment of Functioning (GAF) scale for patients older than 18 and the Child Global Assessment Scale (CGAS) for the patients under 18. Results: The most common symptom, irritability, was accompanied by elated mood in most cases. 73.2% of the patients had at least one comorbid disorder and the most common comorbid disorder was Attention Deficit Hyperactivity Disorder (44.2%). Diagnostic stability in all periods of illness was higher than 80%. Conclusion: This study supports the high diagnostic stability of bipolar disorder in both children and adolescents.