Analyses of patients who died without HSCT with regard to time of death after onset of therapy and type of patient. All patients who died without having a HSCT (n = 75) were divided into 3 groups; (a) verified FHL (verified biallelic mutations = 23, affected sibling = 5), (b) without verified FHL and ≥1-year old at onset (n = 32), and (c) without verified FHL and <1 year old at onset (n = 15). Notably, 10 of 17 (59%) who died days 11 to 28 were aged ≥1 year and without verified FHL as opposed to 3 of 17 (18%) who died the first 10 days (P = .032), and of these 10 patients, 7 had findings possibly associated with toxicity related to overtreatment. Analyzing the treatment period days 11 to 42, this pattern was further strengthened (P = .018). In contrast, among the 8 (4 with verified FHL) who died days 43 to 60, 6 (75%) were reported to suffer active HLH, possibly suggesting a need of less reduction of initial treatment during this period. Finally, 8 of 16 (50%) who died after day 120 had verified FHL, many of whom died of disease reactivation, stressing the importance of an early HSCT for this cohort.