Obesity is common in the equine population and it has been associated with increased surgical morbidity and mortality in humans. The effect of increased body mass index (BMI) on the incidence of surgical site infections has not been investigated in horses. To determine whether horses with increased body fat, as estimated by BMI, are more likely to develop post-operative complications, particularly incisional complications, following emergency ventral midline celiotomy. Retrospective, multi-institutional clinical study. Medical records of horses >2years old presenting with colic that underwent ventral midline celiotomy between January 2010 and September 2018 with follow-up of a minimum of 30 post-operative days were reviewed. Extracted data included signalment, operative details and outcome. BMI was calculated by dividing the patient's weight (kg) by the withers height squared (m2 ). In all, 287 horses fit inclusion criteria. Incisional complication prevalence was 23.7%. Horses with incisional complications had a higher BMI (median 203.6kg/m2 , IQR=191.5-217.4) compared with those without (median 199.1kg/m2 , IQR=184.7-210.2) (P=.03). Multi-variable analysis of the effects of age, sex, breed and presence of metabolic disease on the association between BMI and risk of incisional complications, identified a tendency towards increased risk with a higher BMI, but statistical significance decreased to P=.07. Breed had an association with BMI (P<.01), but not with incisional complication risk. BMI as an estimate of body fat has limitations. Retrospective studies with reliance on owners reporting data and complete medical records is imperfect. When the data were subjected to multi-variable analysis, the trend towards an increased incidence of incisional complications in horses with higher BMI persisted but it was not statistically significant. Higher BMI may increase the risk for the development of incisional complications in horses following emergency ventral midline celiotomy.