In pediatric oncology, there is no evidence-based definition of the temperature limit defining fever (TLDF), which itself is essential for the definition of fever in chemotherapy-induced severe neutropenia (FN). Lowering the TLDF can increase the number of FN episodes diagnosed. This prospective, single center observational study collected data on all temperature measurements, complete blood counts (CBCs), and measures of diagnostics and therapy performed at and after FN diagnosis in pediatric oncology patients using a high standard TLDF (39 °C ear temperature). In 45 FN episodes in 20 patients, 3391 temperature measurements and 318 CBCs, plus information on antibiotics, anti-fungal therapy, antipyretics, blood cultures taken and on discharge were collected. These data can mainly be used to study the influence of virtually lowering the TLDF on diagnostic measures, treatment and length of hospitalization in pediatric FN, which in turn are directly related to costs of FN therapy, and quality of life. This approach can be expanded to include as well different definitions of neutropenia.