Objective: To assess the predictive value of a follicular scoring system for monitoring ovulation induction in polycystic ovary syndrome (PCOS) patients, solely with ultrasound (US). Design: Ultrasound measurements were performed on alternate days to define a serial follicular score for monitoring ovulation induction with hMG alone, as well as GnRH analogue and hMG, in comparison with E(2) concentration obtained on the same day. Setting: Outpatient Infertility Clinic, Department of Obstetrics and Gynecology. Patients: Thirty-four consecutive PCOS patients treated for 63 cycles. Main Outcome Measure: The follicular score was established considering the summation of points obtained after measuring the mean diameter of each follicle > 5 mm, as follows: 5 to 8 mm = 1 point, 9 to 12 mm = 1.5 points, 13 to 16 mm = 2 points, greater than or equal to 17 mm = 3 points. Results: Follicular score correlated positively with E(2) concentrations. A score of greater than or equal to 30 points was associated with E(2) levels of concentration that reached > 1,500 pg/mL (conversion factor to SI unit, 3.671) and could predict ovarian hyperstimulation. A lower follicular score allowed hCG administration. Conclusions: A follicular scoring system may be a safe, simple, and highly efficient method to replace serial E(2) measurements in monitoring ovulation induction. Moreover, ovarian hyperstimulation may be predicted.