The effect of breed and body weight on longevity in the pet dog was analyzed, and a method was developed to standardize the chronological age of dogs in terms of physiological time, using human year equivalents. Mortality data from 23,535 pet dogs were obtained from a computerized data base of North American veterinary teaching hospitals, and the median age at death was determined for pure and mixed breed dogs of different body weight. Body size in the dog was inversely related to longevity. Within each body weight category, the median age at death was lower for pure breed dogs compared with mixed breed dogs. The difference between the standardized physiological ages of mixed breed dogs of the same chronological age in the smallest and largest body weight categories varied from 8 to > 15 years, and between large and small pure breed dogs, the disparity was even greater. Laboratory research to explore the biological basis for these breed and body weight specific differences in life span among dogs may provide additional clues to genetic factors influencing senescence.