Comparison: After the pre-trial screening, eligible subjects with presumed endometriosis and age 18-45 will be consented and randomly assigned to receive either placebo (control) or rosiglitazone, Avandia®, 4 mg daily for 2 weeks. Peritoneal fluid will be collected at the time of surgery and the volume measured. All patients enrolled in the study will have their surgery during the follicular phase of the cycle in order to minimize differences in volume and cytokine concentration due to the cyclical changes. The primary measure will be the peritoneal fluid concentration comparisons of the two groups assessing six different cytokines: interleukin-1 beta, RANTES, tumor necrosis factor-alpha and vascular endothelial growth factor.