Administrative coding of out-of-hospital cardiac arrest (OHCA) is heterogeneous, with the prevalence of noninformative diagnoses uncertain.To characterize the prevalence and type of non-informative diagnoses in a young cardiac arrest population.Hospital discharge diagnoses provided to a statewide OHCA registry were characterised as either 'informative' or 'noninformative.' Informative diagnoses stated an OHCA had occurred or defined OHCA as occurring due to coronary artery disease, cardiomyopathy, channelopathy, definite noncardiac cause, or no known cause. Noninformative diagnoses were blank, stated presenting cardiac rhythm only, provided irrelevant information or presented a complication of the OHCA as the main diagnosis. Characteristics of patients receiving informative versus noninformative diagnoses were compared.Of 1479 patients with OHCA aged 1 to 50 years, 290 patients were admitted to 15 hospitals. Ninety diagnoses (31.0%) were noninformative (arrest rhythm = 50, blank = 21, complication = 10 and irrelevant = 9). Two hundred diagnoses (69.0%) were informative (cardiac arrest = 84, coronary artery disease = 54, noncardiac diagnosis = 48, cardiomyopathy = 8, arrhythmia disorder = 4 and unascertained = 2). Only 10 diagnoses (3.5%) included both OHCA and an underlying cause. Patients receiving a noninformative diagnosis were more likely to have survived OHCA or been referred for forensic assessment (P = 0.011) and had longer median length of stay (9 vs 5 days, P = 0.0019).Almost one third of diagnoses for young patients discharged after an OHCA included neither OHCA nor any underlying cause. Underestimating the burden of OHCA impacts ongoing patient and at-risk family care, data sampling strategies, international statistics and research funding.