BACKGROUND: The comparative effectiveness of nonoperative management (NOM) vs immediate splenectomy (IS) for hemodynamically stable adult patients with grade IV or V blunt splenic injury (BSI) has not been clearly established in the literature. STUDY DESIGN: We performed a retrospective analysis of adult patients, from the 2013 to 2014 American College of Surgeons Trauma Quality Improvement Program (TQIP) Participant Use Data Files, who sustained grade IV or V BSI. Outcomes after IS vs attempted NOM were compared using propensity score analysis in order to adjust for patient- and injury-related variables. RESULTS: Nonoperative management was attempted in 1,489 (52.2%) of 2,746 patients who sustained grade IV or V BSI. Propensity matching techniques resulted in a cohort of 758 IS and NOM patients who were well matched for all known patient- and injury-related variables. In-hospital mortality was not different between the IS and NOM patients (11.5% vs 10.0%, p = 0.33), although IS patients had a higher incidence of infectious complications (21.4% vs 16.9%, p = 0.02). The rate of NOM failure in our sample was 20.1%. Independent predictors of failed NOM included the presence of a bleeding disorder, early blood transfusion requirement, and grade V injury. Splenic artery embolization was associated with a decreased risk of NOM failure. Patients who had failed NOM had a lower in-hospital mortality rate than IS patients (6.4% vs 16.4%, p = 0.004), but required longer hospitalization. CONCLUSIONS: Nonoperative management is as effective as IS for hemodynamically stable adult patients with grade IV or V BSI. The delay in operative intervention that results from failed attempts at NOM does not adversely affect the outcomes of patients who ultimately require splenectomy.