Phototherapy is the standard treatment for hyperbilirubinemia in neonates. In severe cases, phototherapy is combined with immunoglobulin treatment and exchange transfusion.1 Phototherapy has saved the lives of millions from death and disability, since 1957, when it was introduced as a treatment for hyperbilirubinemia.2 Although phototherapy is known to reduce serum bilirubin levels effectively, some unresolved questions remain, related to the effectiveness and safety of phototherapy.3 For example, the efficacy of phototherapy in reducing mortality and morbidity remains unquantified, and it remains unclear whether phototherapy increases the short-term risk of asthma or the long-term risk of cancer.3 Access to valid information on phototherapy is important for research on its effectiveness and safety. The Danish National Patient Registry (DNPR) contains information on disease diagnoses, surgical procedures, in-hospital treatments, and care, including phototherapy.4,5 The Danish Health and Medicines Authority guidelines for registering treatments in the DNPR state that “It is not compulsory to register all treatment and care, but the individual department has the opportunity to register the treatment and care of interest at a clinically meaningful level.”6 Thus, for some treatments and care (including phototherapy), there is little incentive to reporting, which often results in incomplete registration. Here, we explored the nationwide registration of phototherapy in the DNPR. Preliminary analyses showed that phototherapy was evidently under-reported in the DNPR across the nation, except at the Aarhus University Hospital (AUH). Therefore, we analyzed an AUH sub-cohort to explore in detail the proportions of phototherapy registered annually and the clinical characteristics of children registered with phototherapy.