Increasing evidence supports the link between ABO(H) blood group determinants and hemostasis. In particular, the ABO-related different glycosylation patterns of von Willebrand factor strongly influence its clearance and functional levels, and this may contribute to the inter-individual variations in the half-life of infused Factor VIII (FVIII) in hemophilia A (HA) patients. We investigated the role of ABO blood groups in regulating FVIII immunogenicity by evaluating their distribution in patients with severe (FVIII < 1 IU/dL) HA according to inhibitor development and other known relevant factors. In a cohort of Italian severe HA patients (n = 209), the ABO blood group distribution was similar to that in the healthy general population. However, the distribution of inhibitors, developed in 56 patients overall (26.8%), was significantly different in the four ABO phenotypes (O, 18.2%; A, 31.9%; B, 39.1%, AB, 25%; p = 0.033); this difference seemed more pronounced when only high-titer inhibitors (overall, 21.1%) were considered (O, 11.4%; A, 27.7%; B, 34.8%; p = 0.011). Relative risks in O versus non-O blood group were 0.55 (95% CI: 0.33-0.92) and 0.40 (95% CI: 0.21-0.77) for any and high-titer inhibitors, respectively. In a multivariate logistic regression, O blood group was shown to lower (approximately twofold) inhibitor risk, similarly with plasma-derived FVIII, whereas high-risk F8 mutations were associated with increased risk. However, the estimated effect of O blood type on inhibitor development was free from any significant correlation to other covariates, including presence of high-risk F8 mutations and type of replacement FVIII used. In this retrospective cohort of severe hemophiliacs, blood group O appears to protect against inhibitor development, with independent effects from other covariates. Larger prospective studies are needed to confirm this finding and to delve deeper into its pathophysiologic mechanisms.