Objective To compare the efficacy of a sequential step-up/step-down protocol with the conventional low dose step-up and the step-down regimens in patients with polycystic ovary syndrome (PCOS) undergoing IVF. Design Prospective randomized study. Setting Private fertility clinic. Patient(s) Two hundred twenty-five, 30-year-old or younger, women with PCOS. Intervention(s) The low dose step-up, the step-down, and a sequential step-up/step-down regimens were used. Main Outcome Measure(s) Ovarian response, pregnancy (PR) and miscarriage rate. Result(s) Patients in group A (low dose step-up) exhibited the highest cancellation rate, whereas subjects in group C (sequential regimen) the lowest. The hormonal dosage (ampules) and duration of stimulation (days) was higher in group A patients and lower in group B (step-down regimen). The follicular growth uniformity was superior in group C patients who also achieved a higher number of eggs at retrieval, better fertilization and embryo development rates. The sequential group yielded the highest pregnancy and the lowest miscarriage rate. Conclusion(s) The sequential step-up/step-down stimulation protocol is superior to the low dose step-up and step-down regimens with regard to ovarian response, PR and miscarriage rate in 30-year-old or younger patients with PCOS undergoing IVF.