Previous studies did not establish a clear correlation between the presence of endometriosis and the values of body mass index (BMI).The BMI of 366 women with endometriosis was compared to that of 248 controls undergoing laparoscopy because of benign gynaecological conditions. Significant differences at univariate analyses were confirmed by using Analysis of covariance (ANCOVA) to control for potential confounding variables.BMI was significantly lower in women with endometriosis than in controls (p < 0.001); this difference was confirmed when the analysis was restricted to subjects with normal BMI (18.50-24.99 kg/m(2)) (p = 0.002). 4.8% of control subjects and no woman with endometriosis were obese. No significant difference was observed in the BMI of women with mild (revised classification of the American Fertility Society, rAFS I-II) and severe endometriosis (rAFS III-IV).Women with endometriosis have lower BMI and are less frequently obese than control subjects. Further studies should investigate the physiopathological basis of decreased BMI in women with endometriosis.