Fetal ovarian cysts: prenatal ultrasonographic detection and postnatal evaluation and treatment. Ovarian cysts were diagnosed by antenatal ultrasonographic examination in 15 fetuses between 19 and 37 weeks' gestation. In six cases there was ultrasonographic evidence of torsion. Intracystic flocculation, which typically was deposited on the sloping part of the cyst, gave a characteristic liquid interface that was regarded as ultrasonographic evidence of torsion. All cases with evidence of torsion were managed surgically post partum, and in all patients this complication was confirmed. The remaining nine cases were followed up by repeated ultrasonograms, and in all patients disappearance of the cyst was documented within the first 6 months of life. The mean size of cysts with evidence of torsion was 5.41 +/- 0.25 cm, and the mean size of those without torsion was 4.33 +/- 0.3 cm (p less than 0.01). Histologic examination of the surgical specimen in the cases with evidence of torsion revealed follicular cysts in three cases and necrotic ovarian cysts with no specific epithelial findings in the remaining three. We recommend continuous ultrasonographic assessment of antenatally diagnosed cysts and believe that the choice of treatment depends on the appearance of the cyst and its evolution throughout pregnancy. 