To evaluate lateral distribution of endometriosis and ovarian endometrioma in women with endometriosis. We evaluated operative reports of women who underwent laparoscopic treatment of endometriosis (n = 330) and ovarian endometrioma (n = 185) from January 1996 to January 2002. Data on all operative findings consisted of a written report, a diagram, the revised American Fertility Society classification of endometriosis, and a printout of the dictated report. Endometriotic implants were confined to one side of the pelvis in 143 women and bilaterally in 187 others. Endometriosis was significantly more frequent in the left (64.3%) than in the right hemipelvis (P <.001, odds ratio 3.3, 95% confidence interval 2.0, 5.3). Of those with bilateral lesions, adhesions were also more frequently found on the left than on the right hemipelvis (16.6% versus 6.9%, P <.01, odds ratio 2.6, 95% confidence interval 1.3, 5.2). Endometrioma was found in the left ovary (n = 90), in the right ovary (n = 59), and bilaterally (n = 36). Left ovarian endometrioma was found more frequently (60.4%) than right endometrioma (P <.001, odds ratio 2.3, 95% confidence interval 1.5, 3.7). This trend was not related to the size of the endometrioma.Our results confirm a left lateral predisposition of endometriosis and ovarian endometrioma. It is possible that this is related to decreased fluid movement in the left side of the pelvis because of the presence of sigmoid colon. These findings may support the theory that the origin of endometriosis is from the regurgitated endometrial cells.