CONTEXT: The role of human chorionic gonadotropin (hCG) supplementation on the intrafollicular steroid milieu has been studied. OBJECTIVE: The objective of the study was to assess the impact on steroid levels in follicular fluids (FFs) after different doses of hCG supplementation to recombinant FSH for controlled ovarian stimulation. SETTING: This was a prospective randomized dose-response study conducted at Copenhagen University Hospital, Rigshospitalet, Denmark. PATIENTS: From 62 in vitro fertilization patients, 334 FFs were selected for analyses. INTERVENTIONS: Patients were treated using a GnRH agonist protocol with recombinant FSH 150 IU/d and randomized from stimulation day 1 to supplementation with hCG: D0, 0 IU/d; D50, 50 IU/d; D100, 100 IU/d; and D150, 150 IU/d. MAIN OUTCOME MEASURE: Intrafollicular hormone concentrations in relation to treatment groups, follicular sizes, and embryo quality were measured. RESULTS: In large follicles, hCG supplementation induced a nearly 3-fold increase of estradiol (nanomoles per liter) [D0: 1496; D50: 3138; D100: 4338; D150: 4009 (P < .001)], a significant 3-fold increase of androstenedione, and a 5-fold increase of T (nanomoles per liter) [D0: 15; D50: 38; D100: 72; D150: 56 (P < .001)]. The estradiol to T ratio decreased significantly, with the lowest ratio in D100 and the highest in D0. Large follicles giving rise to good-quality embryos had significantly higher estradiol and progesterone levels and estradiol to T, estradiol to androstenedione, and progesterone to estradiol ratios, compared with small follicles, leading to poor-quality embryos. CONCLUSIONS: Increasing doses of hCG supplementation markedly stimulated the intrafollicular concentration of both estradiol and androgens, with a shift toward a more androgenic milieu. In large follicles with oocytes giving rise to good-quality embryos, the FFs were significantly more estrogenic than in small follicles with oocytes developing into poor quality embryos.