In human medicine, congenital heart defects are a well-recognised cause of infant death, but in dogs, they remain largely invisible. Many puppy deaths could be simply labelled as ‘fading puppy syndrome’, but we suspect congenital heart disease may be a major, under-recognised contributor. If this is the case, we may be breeding heritable heart disease into our canine population without even being aware. The impact on puppy welfare could be far larger than we realise.