The 2003 Rotterdam diagnostic criteria for polycystic ovary syndrome (PCOS) support the objective role of ultrasound in defining the appearance of the PCO, but there are significant limitations of these new guidelines from an ultrasound perspective that must be considered. Three-dimensional (3D) ultrasound provides a new method for the objective quantitative assessment of follicle count, ovarian volume, stromal volume and blood flow within the ovary as a whole. Since the introduction of 3D ultrasonography, there have been increasing publications on PCOS, each addressing different areas and reporting different results. This review critically examines these studies in an attempt to clarify the evidence to date and thereby establish the current role of 3D ultrasonography in PCOS.