OBJECTIVE Investigation of the correlation between serum estradiol (E2), salivary E2 and sonographic measurements of follicles in women undergoing controlled ovarian stimulation (COS) for IVF/ICSI. METHODS This is a prospective study performed at the Department of Reproductive Medicine of Ghent University Hospital (Belgium) between November 2016 and January 2017 over a total of 40 patients. During routine COS, two-dimensional measurements of the follicles were performed using transvaginal ultrasound (TVUS) and E2 was measured in saliva and serum. A linear Mixed-Effects model (MIXED) was built, using SPSS Statistics 24. RESULTS Statistical analysis shows a strong linear correlation between serum and salivary E2. For every single unit increase in serum E2 (+ 1 ng/L) the estimated saliva E2 concentration is expected to increase with 0.011 pg/mL (95% CI [0.009 - 0.01]). Strong linear correlations between both saliva and serum E2 and follicular dimensions were also found. For every millimetre increase in follicle diameter the estimated serum E2 concentration is expected to increase with 8.32 ng/L (95% CI [7, 10-9, 54]). For every millimetre increase in follicle diameter the estimated saliva level of E2 is expected to increase with 0.11 pg/mL (95% CI [0.09 - 0.13]). CONCLUSIONS A strong correlation between serum and salivary E2 concentrations was found. In addition, both are strongly correlated with the product of the number of follicles and their average diameter, measured by TVUS. More investigation needs to be done to find out if salivary E2 is an effective tool for monitoring IVF cycles.