Background: The "obesity paradox" exists in many diseases. It is unclear whether it also exists in acute respiratory distress syndrome (ARDS). The purpose of our study was to clarify the relationship between obesity and the development of and hospital mortality from ARDS among patients who underwent cardiac surgery. Material/Methods: This retrospective case-control study included 202 patients with ARDS and 808 matching patients without ARDS. We clarified the relationship between obesity and the development of ARDS after adjusting for confounding factors by multiple logistic regression analysis. A total of 202 ARDS patients were divided into survival and mortality groups. After all confounding factors were adjusted by multiple logistic regression analysis, we demonstrated the relationship between obesity and mortality from ARDS. Results: We found a significant association between body mass index (BMI) and the development of ARDS; the cutoff point of BMI was 24.78 kg/m2 by adjusting for confounding factors for the development of ARDS. When the BMI was lower than 24.78 kg/m2, the higher BMI was a protective factor (odds ratio [OR] 0.68, P=0.000, 95% confidence interval [CI] 0.55-0.84). When the BMI was higher than 24.78 kg/m2, the higher BMI was a risk factor (OR 1.07, P=0.050, 95% CI 1.00-1.14). However, obesity was found to be associated with decreased ARDS mortality by adjusting for confounding factors (OR 0.91, P=0.039, 95% CI 0.83-1.00). Conclusions: An "obesity paradox" may exist in ARDS among patients with obesity who undergo cardiac surgery.