We presented a case of a 13-year-old girl who attended the emergency department with acute urinary retention and 1400 mL residual urine after catheterisation. She had no significant medical history, neurological examination was normal and she had not reached menarche. She was found to have a haematocolpos on ultrasound scan which was compressing the urinary bladder. Examination under anaesthesia confirmed an imperforate hymen and therefore an incision was performed and the haematocolpos drained. She managed to pass urine normally the day following her procedure. In this article, we emphasise on the differential diagnosis in this case and the learning points derived from it.