Paediatric bipolar disorder was first proposed as a research hypothesis in two articles in 1995. Two research groups postulated that mania might occur in early childhood when it manifests differently from adulthood—as multiple mood episodes per day (ultradian cycling) or chronic irritability generally without elevated mood. Paediatric bipolar disorder rapidly entered clinical practice in the USA via the Diagnostic and Statistical Manual of Mental Disorders (DSM) category of bipolar disorder–not otherwise specified. By 2004, bipolar diagnoses in children and adolescents had increased 40 times in US primary health care and become the most common diagnosis in preadolescent inpatient units. Between 2000 and 2010, mean bipolar diagnoses for children aged up to 19 years were 100·9 per 100,000 children in 50 US inpatient units, peaking at 140 per 100,000 children in 2006, then falling back to the mean by 2010.