Mature cystic teratomas (dermoid cysts) are the most common germ cell tumours with 10-25% incidence of adult and 50% of paediatric ovarian tumours. The aetiology of dermoid cysts is still unclear, although currently the parthenogenic theory is most widely accepted. The tumour is slow-growing and in the majority of cases it is an accidental finding. Presenting symptoms are vague and nonspecific. The main complication of a dermoid cyst is cyst torsion (15%); other reported complications include malignant transformation (1-2%), infection (1%), and rupture (0.3-2%). Prolonged pressure during pregnancy, torsion with infarction, or a direct trauma are the main risk factors for a spontaneous dermoid rupture that can lead to acute or chronic peritonitis. The diagnosis of mature cystic teratoma is often made in retrospect after surgical resection of an ovarian cyst, because such imaging modalities as ultrasound, computer tomography, or magnetic resonance imaging cannot yet accurately and reliably distinguish between benign and malignant pathology.