Fidanacogene elaparvovec is an adeno-associated virus (AAV)-based gene therapy that enables endogenous factor IX (FIX) expression in hemophilia B, potentially eliminating the need for prophylactic FIX replacement. Post-treatment monitoring focuses on FIX durability, hepatic safety, and immune-mediated loss of expression. A 40-year-old man with severe hemophilia B received a single intravenous dose of fidanacogene elaparvovec in the phase 3 BENEGENE-2 trial (NCT03861273). FIX activity was assessed by one-stage and chromogenic assays. Liver enzymes, clinical parameters, and adverse events were monitored. Prednisolone was initiated for liver enzyme elevation and later switched to budesonide in response to declining FIX levels. Follow-up continued for 4 years. FIX activity peaked at week 6 (~30 IU/dL) and remained stable until week 16. Moderate ALT elevation (peak ~70 U/L) responded to prednisolone. Between weeks 22–28, FIX levels declined without ALT elevation. Budesonide (9 mg/day) was started at week 25 for 3 months and gradually tapered, stabilizing FIX activity at 10-20 IU/dL. This level has been maintained for 4 years without bleeding episodes or FIX replacement. Fidanacogene elaparvovec provided sustained, clinically meaningful FIX expression. Budesonide may offer an effective hepatocyte-targeted alternative to systemic corticosteroids while supporting long-term transgene expression.