The objective of this study was to evaluate size, bilaterality, histopathologic origin, and the serum levels of some tumor markers in patients with mature cystic teratomas (MCTs) of the ovary. Retrospective study of 80 patients operated at Department of Obstetrics and Gynecology, Gulhane Military Medical Academy, Turkey, between the years 1998 and 2003 was performed. The mean age was 33.1 +/- 11.3 years (median 34; range 15-69). The mean tumor diameter was 7.2 +/- 4.5 cm (median 5; range 3-20). The mean serum CA19-9 level was 101.2 +/- 179.7 IU/mL (range 1-841, normal value < 37), the mean serum CA125 level was 32.0 +/- 37.8 U/mL (range 2.2-205, normal value < 35), the mean carcino-embryogenic antigen (CEA) level was 1.46 +/- 1.20 ng/mL (range 0.2-4.8, normal value 3.4), and the mean serum alpha-fetoprotein (AFP) level was 2.7 +/- 3.0 ng/mL (range 0.1-10.5, normal value <8.1). The elevated rate of CA19-9, CA125, CEA, and AFP was 38.8% (31/80), 25% (18/72), 9.1% (4/44), and 8.7% (4/46), respectively. The bilaterality rate was 27.5% (22/80). Patients with an elevated serum CA19-9 level showed significantly higher bilaterality rate (51.6% versus 12.2%, P < 0.05) than the patients with low levels. Likelihood ratio for bilaterality was 2.8 for CA19-9 and 4.6 for CA125. Bilateral teratomas showed a significantly higher rate of ectodermal component than the unilateral ones (100% versus 74.3%, P < 0.05). Ovarian MCTs were diagnosed especially during the reproductive period. CA19-9 may be the only important marker in the diagnosis of MCTs. Elevated levels of CA19-9 and CA125 may be an indicator of bilaterality. Since levels of CA19-9 and CA125 may be elevated in both benign and malignant conditions, interpretation of these findings must be made in light of the clinical condition of the patient.