Objective: To clarify whether the presence of ovarian endometriomas is associated with a reduced responsiveness to ovarian hyperstimulation. Design: Observational study. Setting: University teaching hospital. Patient(s)- : Women selected for in vitro fertilization intracytoplasmic sperm injection (IVF-ICSI) cycles who were found to have unilateral ovarian endometriomas and who did not undergo previous ovarian surgery. Intervention(s): Ovarian hyperstimulation using gonadotropins. Main Outcome Measure(s): Number of codominant follicles (mean diameter > 15 mm) in the affected and in the contralateral intact gonads. Result(s): Thirty-six patients were enrolled. They underwent 56 IVF-ICSI cycles. The number of codominant follicles in the intact and affected ovaries were 4.0 ± 2.2 and 3.0 ± 1.7, respectively (P = .01)-. This difference corresponded to a mean reduction (95% confidence interval [CI]) of 25% (6% - 44% ). This deleterious effect was more evident in women with larger cysts, in those with more than one cyst, and in those who were more responsive to ovarian hyperstimulation. Conclusion(s): The presence of ovarian endometriomas is associated with a reduced responsiveness to gonadotropins. Objective: To clarify whether the presence of ovarian endometriomas is associated with a reduced responsiveness to ovarian hyperstimulation. Design: Observational study. Setting: University teaching hospital. Patient (s) -: Women selected for in vitro fertilization intracytoplasmic sperm injection (IVF-ICSI ) cycles who were found to have unilateral ovarian endometriomas and who did not survive previous ovarian surgery. Intervention (s): Ovarian hyperstimulation using gonadotropins. Main Outcome Measure (s): Number of codominant follicles (mean diameter> 15 mm) in the affected They underwent 56 IVF-ICSI cycles. The number of codominant follicles in the intact and affected ovaries were 4.0 ± 2.2 and 3.0 ± 1.7, respectively (P = .01) - This difference effect was more evident in women with larger cysts, in (95% confidence interval [CI]) of 25% (6% - 44% those with more than one cyst, and in those who were more responsive to ovarian hyperstimulation. Conclusion (s): The presence of ovarian endometriomas is associated with a reduced responsiveness to gonadotropins.