To describe patient characteristics of dogs developing spontaneous abdominal effusion in association with anaphylaxis. Retrospective study between 2010 and 2018. University teaching hospital and referral specialty private practice emergency departments. Sixteen dogs presenting to the emergency department for clinical signs suggestive of anaphylaxis with peritoneal effusion documented via ultrasonography. None. Dogs presented with clinical signs of anaphylaxis including vomiting 12 of 16 (75%), diarrhea 9 of 16 (56%), and collapse 7 of 16 (44%). Physical examination findings included altered mentation 14 of 16 (88%), erythema 2 of 16 (13%), and urticaria 2 of 16 (13%). Historically, 3 of 16 (19%) had just recently been vaccinated, 1 of 16 (6%) had known contact with an insect, and 8 of 16 (50%) had been outdoors prior to the onset of clinical signs, whereas 5 of 16 (31%) had no known triggering event. At presentation, median systolic arterial pressure was 70mm Hg (range, 30-210mm Hg). Venous blood gas revealed an acidemia with a mean pH of 7.24 ± 0.09, a base deficit of 7.15 ± 7.7mmol/L, and an increased plasma lactate with a mean of 7.35 ± 2.09mmol/L. Bloodwork abnormalities included an increased alanine aminotransferase activity with a mean of 439.2 ± 404.5U/L and an increased gamma-glutamyl transferase activity with a mean concentration of 7.29 ± 4.1U/L. Twelve patients (75%) had gallbladder wall abnormalities on ultrasonography. Eight patients (50%) had abdominocentesis performed. The median PCV of the effusion was 0.29 L/L (29%; range, 4-41) and total plasma protein was 38g/L (3.8g/dL; range, 2.4-6.5). Two dogs (12.5%) were euthanized. The remaining 14 dogs (87.5%) survived to discharge, with a mean hospitalization time of 23.7 ± 16.7hours. Abdominal effusion should be considered a potential sequela of anaphylaxis in dogs. Therefore, serial monitoring for presence or development of spontaneous abdominal effusion should be considered.