INTRODUCTION Imaging of the levator hiatus, the largest potential hernial portal in the human body, requires axial plane imaging by ultrasound or magnetic resonance imaging. The aim of this study is to determine whether 2D translabial ultrasound may identify excessive hiatal distensibility by measuring the anteroposterior (AP) diameter of the hiatus. This may become clinically relevant for risk stratification of women prior to prolapse surgery. METHODS This is a retrospective analysis of 577 women seen at a tertiary urogynaecological unit between May 2008 and September 2010. All women underwent a standardised interview, clinical prolapse assessment (ICS POP-Q) and translabial ultrasound. The hiatal AP diameter was measured at rest, on Valsalva and during pelvic floor muscle contraction. All analyses were performed offline, blinded against clinical data. RESULTS There was a strong statistical relationship between reported prolapse symptoms/pelvic organ descent and hiatal AP diameter on Valsalva (P < 0.001 on chi-squared test for trend). A cut-off of 6 cm of the AP hiatal diameter on Valsalva yielded a specificity of 0.64 and a sensitivity of 0.7 for detecting significant prolapse on ultrasound. CONCLUSION Hiatal ballooning can be diagnosed with 2D translabial ultrasound. We propose that an AP hiatal diameter on Valsalva of up to 5.99 cm be regarded as normal, with 6-<6.5 cm defined as mild, 6.5-<7 cm as 'moderate', 7-<7.5 cm as 'marked' and 7.5 or more as 'severe' ballooning.