Objective: To measure tissue oxygen saturation (StO 2 ) in a population of dogs with naturally occurring shock and to evaluate the relationship of StO 2 with an established veterinary severity scoring system (Acute Patient Physiologic and Laboratory Evaluation) and patient survival.Design: Prospective observational study. Setting: University teaching hospital.Animals: Twenty-five adult dogs presenting in shock, as determined by the presence of hypotension, the calculated shock index, and hyperlactatemia. Interventions:StO 2 was measured prior to any therapeutic interventions. Blood samples were also collected for measurement of plasma lactate, complete blood count, and a serum biochemical profile. Abdominal and thoracic focused assessment with sonography was also performed. Measurements and Main Results: Dogs enrolled in this study had lower mean (±SD) StO 2 values (65.12 ± 17.7%) than previously reported in experimental models of canine hemorrhagic shock.There was a moderate correlation between lower StO 2 and increasing Acute Patient Physiologic and Laboratory Evaluation scores. A single StO 2 value, assessed prior to therapeutic intervention, was not a sensitive predictor of mortality in this population.Conclusions: Dogs with naturally occurring shock have lower mean StO 2 values than those previously reported in dogs with experimentally induced shock. A lower initial StO 2 was associated with worse disease severity but was not a significant predictor of survival in this population. K E Y W O R D Sdogs, hypovolemia, illness severity, oxygenation indices