Pseudoaneurysms of the mitral-aortic intervalvular fibrosa are rare complications that can develop after mitral or aortic valve surgery, endocarditis, or Takayasu arteritis. The optimal timing of surgery to avoid potentially life-threatening complications of pseudoaneurysms has not been established, and watchful waiting has been adopted in specific situations. We describe the case of a 50-year-old man in whom a pseudoaneurysm of the mitral-aortic intervalvular fibrosa developed after aortic root replacement with a homograft. After 13 years of watchful waiting, reoperation was deemed necessary because the pseudoaneurysm had grown to 48 mm and the aortic regurgitation caused by the degenerated homograft had become severe. This case highlights the need for increased awareness of mitral-aortic intervalvular fibrosa pseudoaneurysms and their management.