The sonographic definition of the empty follicle syndrome is presented in a group of 89 consecutive, unmedicated menstrual cycles in women with primary or secondary infertility. The incidence of the empty follicle syndrome was found to be 50%. The incidence increased with age and was independent of gravidity or the type of follicular rupture (or lack of rupture). These data suggest that the empty follicle syndrome may represent a significant etiologic factor in infertility or other reproductive abnormalities.