OBJECTIVES: Many measures have been developed to assess the health-related quality of life (hrQoL) in humans but few have been developed for animals. Our goal was to develop a QoL instrument , completed by pet owners, that was able to reliably detect changes in QoL in healthy dogs as they age. METHODS: An hrQoL tool was built with input from pet owners and veterinarians. The prototype QoL tool was tested with 167 pet owners of healthy dogs. A second survey was completed approximately two weeks later. Each pet owner was allowed to self-select their dog for inclusion, based on their personal assessment of the pet’s health. The pet owners were veterinarian (n=34) and non-veterinarian (n=133) employees of Pfizer Animal Health. The validation process reduced the tool to 15 items in four domains (happiness, physical functioning, hygiene and mental status) and a single hrQoL assessment. The proposed hrQoL measure is brief (one page), has good known-groups and convergent validity, reliability and high internal consistency.  RESULTS: When dogs were blocked by age into 3 year increments, the QOL score provided by the pet owner was high and essentially unchanged for the first 9 years. The pet owner QoL assessment dropped dramatically for dogs >10 years of age, mirroring the realization that the dog was “slowing down”.  A calculated hrQoL score, derived from the component analysis, demonstrated a statistically significant  (P<0.0001) and near-linear decline across age blocks as the dogs aged. A component analysis of many domains was also able to demonstrate a similar uniform age-related decline. CONCLUSIONS: Quality of life scoring can be used to help guide health care decisions for dogs as they age. Compared to the pet owner-derived score, using an hrQoL score derived from component analysis seems to be more reflective of the gradual age-related changes in a healthy dog.