BACKGROUND: The aim of the present study was to determine serum and follicular fluid C‐reactive protein (CRP) levels in patients undergoing controlled ovarian hyperstimulation (COH) for IVF‐embryo transfer cycle, and their possible correlation to COH variables. PATIENTS AND METHODS: The subjects were 16 consecutive patients undergoing our routine IVF long GnRH agonist protocol. Blood was drawn three times during the COH cycle: (i) the day on which adequate suppression was obtained (Day‐S); (ii) the day of, or prior to HCG administration (Day‐HCG); and (iii) the day of (and before) oocyte pick‐up (Day‐OPU). Levels of sex steroids and serum and follicular fluid CRP were compared among the three time points. Serum and follicular fluid CRP were measured with a commercial immunoturbidimetric assay. RESULTS: Serum levels of CRP were significantly higher on Day‐OPU and Day‐HCG than on Day‐S, and significantly higher on Day‐OPU than on Day‐HCG. No difference was observed between follicular and serum CRP levels on Day‐OPU. No significant correlations were found between serum and follicular fluid CRP, or between serum CRP‐to‐BMI ratio and serum sex steroid levels or IVF treatment variables. CONCLUSIONS: The significant increase in serum CRP levels during COH, especially after HCG administration, suggests that COH potentiates a state of systemic inflammation.