BACKGROUND : Atypical haemolytic uraemic syndrome (aHUS) is a rare, potentially life-threatening condition caused by dysregulation of the complement pathway. Eculizumab is currently the only approved treatment for this disorder. OBJECTIVE : Our objective was to investigate the impact of early administration of eculizumab on inpatient resource use and hospitalisation costs in 222 patients with aHUS. METHODS : We conducted a retrospective analysis of the Premier Perspective® Hospital Database, including patients with a diagnosis of aHUS and evidence of eculizumab use for aHUS. Early initiation was defined as having received eculizumab within 7 days of admission, with late initiation defined as starting eculizumab on day 8 or later. This date represents the average time required to obtain a specific diagnostic test to discriminate aHUS from a similar haemolytic syndrome that requires a different treatment. Outcome measures were time from first eculizumab initiation to discharge, discharge status or death, days spent in the intensive care unit (ICU), readmission indicators, dialysis indicators, and total hospital costs. Time from first eculizumab initiation to discharge was analysed using a generalised linear model with a log link and an assumed underlying negative binomial distribution. Logistic regression models were used to test the statistical significance of early versus late initiation as a predictor of the occurrence of readmissions, dialysis, and death. Total hospital costs were analysed using a generalised linear model with a log link and an assumed underlying gamma distribution. RESULTS : Before modelling, total length of stay and ICU duration were significantly longer for late initiators than for early initiators, and significantly more late initiators were readmitted within 90 days. Late initiation was associated with significantly higher hospital costs than early initiation. After multivariable analysis, late initiators were 3.2 times more likely to require dialysis. However, there was no significant association between early initiation and time to discharge, readmission, or death for any definition or early initiation after multivariable analysis. Estimated total hospital costs (year 2017 values) were $US103,557 in late initiators and $US85,776 in early initiators (p = 0.0024). CONCLUSIONS : Initiation of eculizumab within 7 days of hospitalisation is associated with lower dialysis rates, less time in ICU, less plasmapheresis, and lower hospitalisation costs compared with late initiation.