Background: Severe cholecystitis occurs more frequently in males, and the cause of this discrepancy is unknown. Methods: A retrospective study was performed on 910 consecutive patients who underwent laparoscopic cholecystectomy for cholecystitis. The patients were classified according to body mass index (BMI) as obese (n = 354, BMI ≥25 kg/m2) and nonobese (n = 556; BMI <25). Results: In males, there was a significant negative correlation between the BMI and the severity of cholecystitis; the proportion of complicated acute cholecystitis was higher in the nonobese patients (21.5%) compared with the obese patients (8.1%) (P = .007) but not for the females (P = .80). A BMI <25 (odds ratio [OR] = 1.92, P = .01), advanced age (OR = 2.52, P < .001), male sex (OR = 1.74, P = .022), and leukocytosis (OR = 1.92, P = .024) were independent predictors for the development of complicated acute cholecystitis. Conclusion: There is a negative association between BMI and the inflammation severity of cholecystitis in males, which resulted in a higher incidence of severe cholecystitis in the nonobese male patients.