The CANAL Study (Concerted Action on Neutralizing Antibodies in severe hemophilia A) was designed to describe the relationship between treatment characteristics and inhibitor development in previously untreated patients with severe hemophilia A. This multicenter retrospective cohort study investigated 366 consecutive patients born between 1990 and 2000. The outcome was clinically relevant inhibitor development, defined as the occurrence of at least 2 positive inhibitor titers combined with a decreased recovery. Eighty-seven (24%) patients developed inhibitors (69 high titer [19%]). The incidence of inhibitors appeared to be associated with age at first treatment, decreasing from 41% for those treated within the first month of age to 18% in those treated after 18 months; after adjustment for treatment intensity, this association largely disappeared. Surgical procedures and peak treatment moments at start of treatment increased inhibitor risk (relative risk [RR], 3.7; 95% confidence interval [CI], 2.0-7.1; and RR, 3.3; CI, 2.1-5.3, respectively). Regular prophylaxis was associated with a 60% lower risk than on-demand treatment (RR, 0.4; CI, 0.2-0.8). Our findings suggest that the previously reported associated between an early age at first exposure and the risk of inhibitor development is largely explained by early, intensive treatment. The latter appears to be an independent risk factor for inhibitor development. In addition, early, regular prophylaxis may protect patients with hemophilia against the development of inhibitors.