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Objective: To evaluate the effect of 5 cm H 2 O of continuous positive airway pressure (CPAP) on laryngeal size in spontaneously breathing anesthetized dogs via computed tomography (CT)., Design: Prospective, randomized, cross-over clinical study., Setting: University teaching hospital and referral private practice., Animals: Eight healthy client-owned dogs undergoing CT., Interventions: Dogs were sedated with acepromazine 20 μg/kg IM and induced with fentanyl 2 μg/kg and propofol 3-5 mg/kg IV before being maintained on fentanyl (5 μg/kg/h) and propofol (0.3 mg/kg/min) constant rate infusion. Dogs received an air/oxygen mixture with (CPAP) and without (NO-CPAP) 5 cm H 2 O of CPAP in a random order. Each study step lasted 15 minutes., Measurements and Main Results: Ten minutes after the beginning of each study period, a CT scan of the laryngeal region was obtained at end-expiration. CT images were analyzed to determine the laryngeal cross-sectional area (CSA; cm 2 ), total volume (V TOT ; cm 3 ), and laterolateral and dorsoventral diameters (D LL and D DV , respectively; cm). Differences between the 2 treatments were analyzed with t-test for paired data (P < 0.05). Compared to the NO-CPAP, during CPAP the CSA increased by 53.3 ± 23.1% (ie, from 3.3 ± 0.8 to 5.1 ± 1.3 cm 2 , P = 0.0004), V TOT increased by 52.4 ± 13.6% (from 6.2 ± 1.7 to 9.4 ± 2.4 cm 3 , P < 0.0001), and D LL and D DV were 55.5 ± 13.3% (3.6 ± 0.8 vs 2.4 ± 0.5 cm, P = 0.006) and 20.3 ± 8.8% larger (3.2 ± 0.7 vs 2.7 ± 0.6 cm, P = 0.0002), respectively., Conclusions: Laryngeal volume and cross sectional area increased during the application of 5 cm H 2 O of helmet CPAP in spontaneously breathing anesthetized dogs., (© Veterinary Emergency and Critical Care Society 2020.)