A 62-year-old man with a medical history of aortic valve replacement was referred to our hospital with high-grade fever. Blood culture was positive for Streptococcus dysgalactiae, and the echocardiogram showed edematous aortic annulus, suggesting a perivalvular abscess. Treatment with antibiotics was started, which showed progressive improvement. The echocardiogram at 2 weeks after admission showed progression of the perivalvular abscess, resulting in the formation of a perivalvular pseudoaneruysm, which revealed rapid enlargement. The patient underwent surgical resection of a 20-mm pseudoaneurysm, originating from the right and left coronary cusp. Complete resection of the infective tissue was performed, and an aortic root replacement was done. This case highlights that a frequent follow-up should be performed in case of perivalvular abscess, because of the risk of pseudoaneurysm formation, which may cause a life-threatening outcome.