Orvieto R1, Ben-Rafael Z, Schwartz A, Abir R, Fisch B, La-Marca A, Bar-Hava I. Abstract We sought to determine whether neutrophil activation, as reflected by soluble L-selectin levels, plays a role in controlled ovarian hyperstimulation (COH) and the possible correlation between soluble L-selectin and serum sex steroid levels. The study population consisted of 14 consecutive patients undergoing our routine in vitro fertilization (IVF) long gonadotropin-releasing hormone (GnRH) analog protocol. Blood was drawn three times during the COH cycle: (1) on the day when adequate suppression was obtained (Day-S); (2) on the day of, or the day prior to, human chorionic gonadotropin (hCG) administration (Day-hCG); and (3) on the day of ovum pick-up (Day-OPU). Levels of sex steroids and plasma soluble leukocyte selectin (L-selectin) were compared among the three time points. Soluble L-selectin was measured with a commercial sandwich enzyme-linked immunosorbent assay (ELISA). The results showed significantly higher levels of soluble L-selectin on Day-OPU than on Day-S and Day-hCG, and significantly lower levels on Day-hCG than Day-S. Though no significant correlations were found between soluble L-selectin and serum estradiol or hCG levels, soluble L-selectin positively correlated with serum progesterone levels. We conclude that hCG administration leads to neutrophil activation, which correlates with the degree of luteinization. Further studies are required to elucidate the relationship between the immune system and COH. These may lead to new strategies for predicting and preventing complications of COH.