Background Insufficient information is available on the relationship between obesity and outcome of paediatric patients with acute pancreatitis.

Objectives This study aimed to investigate the effect of obesity on outcomes of paediatric patients with acute pancreatitis based on a national administrative database.

Methods A total of 500 cases in 416 paediatric patients with acute pancreatitis (aged 5–17 years) were referred from 260 hospitals between 2010 and 2012 in Japan. Patients were divided into two groups according to the presence of obesity: with obesity (n = 65) and without obesity (n = 435). Patient data were collected from the administrative database to compare the prevalence of severe acute pancreatitis, in‐hospital mortality, length of stay (LOS) and medical costs between the groups.

Results Both prevalence of severe acute pancreatitis and in‐hospital mortality were significantly higher in paediatric patients with obesity than those without (36.9% vs. 16.3% and 3.1% vs. 0.0%; P < 0.001, respectively). Longer LOS and higher medical costs were also observed in paediatric patients with obesity (25.7 vs. 15.2 days, P < 0.001 and 14 169.5 vs. 7457.7 US dollars, P < 0.001, respectively).