Purpose There is no information available about the IL-18 receptor in ovarian follicles, so the present study attempts to demonstrate the expression of IL-18 and its receptor in human granulosa cells (GCs). Methods To evaluate the concentration of IL-18 in serum and follicular fluid (FF), we collected serum and FF from 102 women undergoing oocyte retrieval. Also, to detect expression of IL-18 and its receptor by luteinized GCs, these cells were pooled six times from a total of twenty individual patients with 5-16 follicles each. The IL-18 concentration was determined by ELISA and the expression of IL-18 and its receptor by immunocytochemistry and reverse transcription polymerase chain reaction. Results Our results showed that the median IL-18 concentration in serum, 159.27 pg/ml (IQR 121.41-210.1), was significantly higher than in FF, 142.1 pg/ml (IQR 95.7176.5), p < 0.001. Moreover, we found that IL-18 and its receptor are expressed by GCs. Conclusion The presence of IL-18 in FF and the expression of IL-18 and its receptor by GCs suggest an important role for this cytokine in ovarian function.