OBJECTIVES: To assess the usefulness of clinical and clinicopathological parameters as prognostic markers of survival in dogs undergoing positive pressure ventilation. METHODS: Retrospective study of case records of 58 client-owned dogs undergoing positive pressure ventilation. Dogs were divided into two groups; inadequate oxygenation due to pulmonary parenchymal disease (Group 1) and inadequate ventilation (Group 2). RESULTS: Median duration of positive pressure ventilation was 30 (range 10 to 136) hours. Survival rate was 32% (19 dogs). Survivors were significantly younger (P<0·005) and had significantly higher (P<0·002) median PaO2/FiO2 ratio at 4 to 12 hours postinitiation of positive pressure ventilation, and immediately before weaning (P<0·006) compared to non-survivors. A receiver operator characteristics analysis of PaO2/FiO2 immediately before weaning as predictor of survival had an area under the curve of 0·76 (95% confidence interval 0·54 to 0·97), with optimal cut-off point of 252 mmHg, corresponding to a sensitivity and specificity of 0·80 and 0·79, respectively. The survival rates of dogs with PaO2/FiO2 less than 200 mmHg at 4 to 12 hours postinitiation of positive pressure ventilation, or immediately before weaning were 15% (3/20 dogs) and 6% (1/16 dogs), respectively. CLINICAL SIGNIFICANCE: The PaO2/FiO2 ratio is an early prognostic indicator of successful weaning in dogs undergoing positive pressure ventilation.