"Wasting" or "fading" syndromes are common causes of puppy and kitten mortality. Numerous infectious and toxic, metabolic, or nutritional factors could potentially be responsible for wasting and death in young animals. Evidence has been presented that infectious canine hepatitis virus infection, beta-hemolytic streptococcus infection, and feline infectious peritonitis virus infection are responsible for a significant number of deaths due to wasting syndrome. However, many cases of wasting syndrome cannot be attributed to infectious agents or other specific etiologies. The thymus gland warrants special attention when one is evaluating an animal with a wasting syndrome because it is known that, in some species, neonatal thymectomy results in wasting and death. Unfortunately, most reports describing fading syndromes in puppies and kittens do not mention the gross or histologic appearance of the thymus gland at postmortem examination. When examining the thymus gland, one must keep in mind that the thymus may be hypoplastic owing to a congenital or genetic defect in its structure and function or it may be atrophic secondary to whatever is causing the fading syndrome. If a thorough history, clinical examination, and/or postmortem examination do not reveal a cause for the fading syndrome, then defective thymus function should be considered as a possible causative or contributing factor to the fading syndrome. In these cases, therapy designed to replace or improve the defective thymus function should be considered. At least one form of wasting syndrome in puppies (immunodeficient dwarfism) has been found to respond to short-term therapy with a thymus hormone (thymosin fraction 5) or with bovine growth hormone (which is thymotropic) in limited clinical trials. It is possible that other forms of wasting or fading syndromes would also respond to therapy with thymus hormone or growth hormone. Certain thymus hormones (thymopoietin pentapeptide, thymosin alpha 1, facteur thymique serique, and rabbit thymus acetone powder) and bovine growth hormone are commercially available. Before initiating therapy, one should consider that if the cause of the wasting syndrome is genetic, then successful treatment may perpetuate a genetic defect. More research (both basic and clinical) is needed to determine the role of thymus gland dysfunction in fading syndromes of puppies and kittens and if therapy with one or several of the thymus hormones or with growth hormone could reverse the symptoms of wasting.