Several workers have reported rapid changes in cervical length or cervical dilatation during transabdominal ultrasonographic assessment. Here we describe a case of cervical incompetency in which the same findings were demonstrated during transvaginal ultrasonographic assessment. A primigravid patient suspected of cervical incompetency was referred to this institution at 25 weeks. Although she had felt no painful uterine contraction, transvaginal ultrasonographic assessment revealed a shortened cervix and widely dilated internal os. Transvaginal ultrasonographic examination conducted 3 hours after hospitalization showed the cervix to be anatomically normal with an internal os that appeared to be closed. Funneling of the internal os with protrusion of the membranes into the endocervical canal began after application of transfundal pressure, and the length of remained endocervix was 19 mm 1 minute later. The patient was treated successfully with cerclage placement, and the cerclage was removed at 36.5 weeks. At 39 weeks the patient was admitted in labor and delivered a 3270 g female infant with an Apgar score of 9 at 1 minute.