The empty follicle syndrome was assessed using transvaginal ultrasonography in a group of 152 consecutive women with unmedicated menstrual cycles being studied because of primary or secondary infertility or repetitive miscarriage. The overall frequency of the empty follicle syndrome was found to be 43.4%. The frequency increased with age but was independent of gravidity. The empty follicle syndrome may be a significant etiologic factor in infertility or other reproductive abnormalities, and transvaginal ultrasound represents a good, non-invasive means of evaluating it.