Canine pituitary-dependent hyperadrenocorticism (PDH), or Cushing’s disease, is a severe and frequent endocrine disease in middle-aged to elderly dogs. Small breeds are over-represented and some breeds are at a higher risk of developing the disease than others. PDH is caused by an ACTH-producing pituitary adenoma with unknown pathogenesis. At the Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine of the University of Utrecht there is a long term commitment to study pituitary adenomas and it is the single veterinary institution where transsphenoidal hypophysectomies are performed as routine treatment for canine PDH. This study will lead to targeted studies with large scale genome, cDNA, and protein arrays to elucidate the pathogenesis in dogs.