We describe a case of a 2-week-old male infant who presented with a rapidly enlarging inguinal mass after having received both the bacille Calmette-Guérin (BCG) and hepatitis B vaccines at birth. The clinical picture raised suspicion of a neoplasm, and an excision biopsy was performed. It showed complete effacement of the lymph node architecture by a diffuse proliferation of monomorphic, mitotically active, and medium-sized T-cell blasts with strong expression of CD99. Coalescent necrotizing granulomas were also seen. The lymph node culture was negative for BCG. Upon expert review and additional molecular diagnostics, the initial pathological diagnosis of lymphoblastic T-cell lymphoma was changed to ectopic BCG lymphadenitis and hyperimmune post-vaccinal reaction. The atypical T-cell proliferation was most likely a result of the adjuvant effects of the co-administered vaccines. Post-vaccinal reactions usually involve the injection site or result in localized lymph node enlargements in the areas draining the inoculation site. This case highlights the importance of the clinical context for accurate interpretation of the pathological findings. In the setting of post-vaccinal lymphadenopathy, a biopsy is rarely needed but, when performed, should be interpreted with great caution.