Purpose: The aim of this study was to investigate prophylactic treatment effects in Korean patients with severe hemophilia A. Methods: A prospective study of 32 severe hemophilia A patients was conducted with the approval of the Institutional Review Board at the Eulji University Hospital. Two patients received primary prophylaxis; whereas, the other 30 patients were divided into 2 groups-secondary prophylaxis (n=15) and on-demand (n=15)-on the basis of their consent for secondary prophylaxis. A 20-25 IU/kg dose of factor VIII concentrate was administered to the primary and secondary prophylaxis group patients every 3 days for 1 year. The prophylactic effect was evaluated by observing changes in the Pettersson scores, annual number of total and joint bleeds, and factor VIII consumption for 1 year. Results: No moderate or severe bleeding was observed, and the Pettersson scores remained unchanged during the prophylaxis period in the patients who received primary prophylactic treatment. After the treatment was changed from on-demand to secondary prophylaxis, the annual number of total and joint bleeds in the secondary prophylaxis group decreased by $64.4%{pm}13.0%$ and $70.0%{pm}15.2%$, respectively. The average increase in Pettersson scores within 1 year was $0.5{pm}0.8$ and $1.3{pm}1.1$ in the secondary prophylaxis and on-demand groups, respectively. Prophylactic effects were also observed in patients >17 years who had nearly the same initial Pettersson scores. Conclusion: Intermediate-dose prophylactic treatment may delay hemarthropathy progression and prevent its occurrence in Korean severe hemophilia A patients.