The aim of this study was to evaluate whether antral follicle size has any value with respect to antral follicle count (AFC) in predicting ovarian response to controlled ovarian stimulation. Patients who were considered to be normal responders based on their ovarian reserve markers were consecutively recruited. Total AFC was the number of 2-10 mm follicles in both ovaries. Antral follicles were separated into two subgroups according to the size: 2-5 mm and 6-10 mm follicles. Patients were divided into two main groups according to ovarian response to COS. Group 1 (suboptimal response, 4-9 oocytes retrieved, n = 61) and Group 2 (normal responders, >= 10 oocytes retrieved, n = 65). Demographic parameters were comparable between the groups. The number of 2-5 mm follicles was significantly lower in the suboptimal response group (5 [4:7] and 8 [6:10], respectively, p < .001). The ratio of 2-5 mm follicles to total antral follicles was also significantly lower in Group 1 (44.4% and 75%, respectively, p < .001). The ratio of small antral follicles was positively correlated with ovarian response (r = 0.587, p < .001). In conclusion, the ratio of small (2-5 mm) antral follicles could be a more specific predictive marker than AFC for ovarian response.IMPACT STATEMENT What is already known on this subject? Prediction of the ovarian response during ovarian stimulation is commonly based on antral follicle count and anti-mullerian hormone. The ovarian response may be inadequate even in patients with normal antral follicle count and anti-mullerian hormone. What do the results of this study add? A high ratio of small-size (2-5 mm) basal antral follicles is a predictive factor for higher ovarian response to ovarian hyperstimulation. What are the implications of these findings for clinical practice and/or further research? To obtain optimal ovarian response, the antral follicles should be evaluated initially in a more detailed and systematic way by taking their sizes into consideration in addition to their counts. Small antral follicle count rather than whole antral follicle count may be beneficial for optimising the ovarian response. Future studies may determine the cut off values of small antral follicle count for high/poor ovarian response.