Objective To investigate whether asymmetry exists in the left-and right-handed distribution of Design Retrospective evaluation of a case series. Setting Tertiary care and referral academic centre for the study and treatment of endometriosis. Population A total of 1054 consecutive women undergoing first-line surgical treatment for Methods Data were collected on indication for the intervention, age at surgery, parity and disease Main outcome measure Frequency of left-and right-sided ovarian endometriomas. Results Histologically confirmed endometriotic ovarian cysts were present in 561 women, which were on the left side in 255 instances, on the right in 148, and bilateral in 158. In the patients with unilateral endometriomas, the observed proportion of left cysts (255/403, 63%; 95% confidence interval, 58Y0 to 68Yo) was significantly different from the expected proportion of 50% (x2,, 28.41, P