Clinical and pathologic features of cyclic hematopoiesis in 18 grey collie dogs, aged 10 to 113 weeks, were reviewed. The dogs were grouped according to weeks of age: 10-16 (I), 17-21 (II), 30-35 (III), and less than 52 (IV). Clinical illness occurring during each hematopoietic cycle was classified as none, mild, moderate, or severe, based on the neutrophil count, rectal temperature, clinical signs, and use of antimicrobial therapy. The dogs in Groups I, III, and IV had severe infections episodes during one-fourth of all hematopoietic cycles; whereas the dogs in Group II had severe infections during two-thirds of cycles. However, during the cycle prior to death, all groups were similar, each having two-thirds of clinical syndromes classified as severe and one-third as mild. More dogs died during the neutropenic phase of the hematopoietic cycle than during the nonneutropenic phase. Pathologic findings showed distinct patterns in relation to age. Younger dogs showed evidence of acute infectious processes, especially in the lungs, gastrointestinal tract, and kidneys; whereas older dogs had chronic inflammatory changes in those organs. Amyloidosis was a prominent finding in dogs over 30 weeks of age. These findings indicate that predictable age-related changes in tissues of grey collie dogs impair various organ systems and thereby contribute to morbidity and mortality in older dogs. consequently, future clinical and pathologic studies of grey collies should take into consideration the age of the dogs under study.