Objective: To evaluate bilaterality, complications and malignant changes of mature cystic teratomas of the ovary. Study design. Retrospective study of 501 patients operated at Hacettepe University Hospital between the years of 1964 and 1998. Results: The median age was 35 years (range 13-76). One hundred and six cases (21.1%) were asymptomatic. The mean tumor diameter was 7.0+/-4.5 cm, The decision for cystectomy or oophorectomy was related with the patient age, gravidity and parity. The bilaterality rate when both ovaries were evaluated histopathologically was 13.2% (44/331), Total complication rate was 10.7%, torsion being the most frequent (4.9%). The rate of malignant transformation was 1.4%. Conclusion: Ovarian mature cystic teratomas are common tumors especially during the reproductive period with low rates of covert bilaterality, complications and malignant transformation. The treatment should be directed on the basis of age, fertility desire or presence of another pelvic pathology rather than the size or bilaterality. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.