OBJECTIVE To investigate the role of interleukin-12 (IL-12) and IL-8 in the periovulatory follicular fluid during in vitro fertilization cycles. METHODS A prospective study. METHODS Reproductive Medicine Unit, Liverpool Women's Hospital, United Kingdom. METHODS Women undergoing in vitro fertilization treatment. METHODS IL-8 and IL-12 concentrations in follicular fluid samples that had been collected during transvaginal oocyte retrieval were measured using an enzyme-linked immunosorbent assay (ELISA). Cytokine concentrations were correlated to fertilization rates and treatment outcome. METHODS Fertilization rates and ultrasonographic evidence of intrauterine pregnancy by 4 weeks after embryo transfer. RESULTS Failed fertilization in women with detectable IL-12 was significantly higher (45.5%) than in the IL-12 negative group (6.1%), P=.01. None of the women with detectable IL-12 achieved a pregnancy at the end of the treatment (P=.01). IL-8 was present in the follicular fluid of all women, and no difference in its concentrations was found between the pregnant and nonpregnant groups. No correlation was found between the follicular fluid concentrations of IL-8 and fertilization rates. CONCLUSIONS The presence of IL-12 in the follicular fluid appears to be associated with a negative outcome in IVF treatment. Interleukin-8 appears to be an essential part of folliculogenesis, although its concentration is not associated with fertilization or implantation rates.