We report a newborn girl with McKusick-Kaufman syndrome who presented at birth with severe life-threatening abdominal distension due to hydrometrocolpos. In children with polydactyly and cystic masses in the lower abdomen should hydrometrocolpos be especially considered. Ultrasonography is of great value in the correct assignment of lower abdominal tumours, but should be completed by vagino(cysto)scopy when hydrometrocolpos is suspected.