A high frequency of occurrence of a wasting disease, unthriftiness, and retarded growth was observed in a group of inbred Weimaraner dogs. Affected pups had a small thymus gland, with a marked absence of thymic cortex. A litter of eight pups from a sire and dam that were known to have produced affected offspring was chosen for further study. The pups had normal concentrations of WBC and gamma-globulins and were able to produce antibody in response to Brucella abortus. Two pups in the litter developed a wasting syndrome and responded well to therapy with thymosin fraction 5. One pup that survived the wasting syndrome had a significant (P < 0.05) depression of its lymphocyte blastogenic response to phytohemagglutinin compared with its surviving littermates. Pups from this litter also lacked a normal increase in plasma growth hormone concentration after the injection of clonidine HCl. These pups had concurrent abnormalities of the thymus-dependent immune function and in growth hormone metabolism. The syndrome in these pups has some features in common with the syndrome in the Ames or Snell-Bagg strains of immunodeficient dwarf mice.