BACKGROUND: The diagnostic stability of pediatric bipolar disorder has not been investigated previously. The aim was to investigate the diagnostic stability of the ICD-10 diagnosis of pediatric mania/bipolar disorder.METHODS: All patients below 19 years of age who got a diagnosis of mania/bipolar disorder at least once in a period from 1994 to 2012 at psychiatric inpatient or outpatient contact in Denmark were identified in a nationwide register.RESULTS: Totally, 354 children and adolescents got a diagnosis of mania/bipolar disorder at least once; a minority, 144 patients (40.7%) got the diagnosis at the first contact whereas the remaining patients (210; 59.3%) got the diagnosis at later contacts before age 19. For the latter patients, the median time elapsed from first treatment contact with the psychiatric service system to the first diagnosis with a manic episode/bipolar disorder was nearly 1 year and for 25% of those patients it took more than 2½ years before the diagnosis was made. The most prevalent other diagnoses than bipolar disorder at first contact were depressive disorder (21.4%), acute and transient psychotic disorders or other non-organic psychosis (19.2%), reaction to stress or adjustment disorder (14.8%) and behavioral and emotional disorders with onset during childhood or adolescents (10.9%). Prevalence rates of schizophrenia, personality disorders, anxiety disorder or hyperkinetic disorders (ADHD) were low.LIMITATIONS: Data concern patients who get contact to hospital psychiatry only.CONCLUSIONS: Clinicians should be more observant on manic symptoms in children and adolescents who at first glance present with transient psychosis, reaction to stress/adjustment disorder or with behavioral and emotional disorders with onset during childhood or adolescents (F90-98) and follow these patients more closely over time identifying putable hypomanic and manic symptoms as early as possible.