Background: Paediatric bipolar disorder (BD) has gained validity, and substantial research in the last 20 years has dissipated the controversy surrounding it. However, data on the prevalence, prodromes, phenomenology, and longitudinal stability of paediatric BD are still required. Methods: We reviewed the medical records of all patients (n=72) with DSM-IV BD evaluated over a 15-year period. We assessed the most frequently present symptoms prior to and at the time of diagnosis, the diagnostic stability of the disorder and its subtypes (I, II and NOS). Results: Patients [75% boys, median age (interquartile range, IQR) at diagnosis 12.6 (9.6-15.7) years] underwent follow up for a median period of 3.86 (1.8-5.9) years. There was a median delay from symptom onset to diagnosis of 2.3 (1.2-4.8) years. At the time of diagnosis, 37.5% had BD-I, 8.3% BD-II, and 54.2% BD-NOS. At follow-up, 62.5% had BD-I, 8.3% had BD-II, and 23.6% had BD-NOS, whereas 4.2% no longer met the DSM-IV criteria for BD. Limitations: Our sample size limited the BD subtype analyses. Some of information was collected retrospectively. Conclusion: 95.8% of our sample retained a BD diagnosis after a median follow-up period of 3.86 years. BD-I patients maintained their diagnosis and subtype of BD. Half of all patients with baseline BD-NOS maintained their BD subtype, but most of the other half showed conversion to BP-I at follow up. Only 4.2% of the sample (all with BD-NOS at baseline) did not meet criteria for BD at follow up, and these patients were stable.