Aims: To determine serum retinol-binding rotein 4 (RBP-4) levels in polycystic ovary syndrome (PCOS) patients undergoing controlled ovarian hyperstimulation (COH) for an in vitro fertilization-embryo transfer (IVF-ET) cycle and the possible correlation to COH variables. Patients and Methods: 11 consecutive PCOS patients undergoing our routine IVF flexible multidose gonadotropin-releasing hormone (GnRH)-antagonist protocol. Blood was drawn three times during the COH cycle: (1) day 1 or 2 of menstruation, and prior to gonadotropin administration (Day-S) (Day-S); (2) day of or prior to human chorionic gonadotropin (hCG) administration (Day-hCG); and (3) day of ovum pick-up (Day-OPU). Levels of estradiol and serum RBP-4 were compared among the three time points. Serum RBP-4 was measured with a commercial immunoassay. Results: Results showed significantly lower levels of serum RBP-4 on Day- OPU and Day-hCG than on Day-S. Though significant correlations were observed between serum RBP-4 and body mass index, fasting glucose or glucose to insulin ratio, no correlations were found between serum RBP-4 and IVF treatment variables or pregnancy rate. Conclusion: While serum RBP-4 decreases during COH for IVF, there is apparently no correlation of serum RBP-4 levels with IVF treatment variables or outcome.