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Objective To identify criteria useful for differentiating closed from open spina bifida antenatally. Patients and methods A retrospective study of cases of spina bifida diagnosed in a referral center between 1997 and 2004. Results Of 66 cases of fetal spina bifida diagnosed at a median gestational age of 21 (range, 16-34) weeks, detailed follow-up was available for 57. Of these, open defects were found in 53 (93.0%) and closed defects in four (7.0%). Closed spina bifida was associated in two cases with a posterior cystic mass with thick walls and a complex appearance, while in two cases the spinal lesion could not be clearly differentiated from an open defect, particularly at mid-gestation. Open spina bifida was always associated with typical alterations of cranial anatomy, including the so-called `banana' and `lemon' signs, while in closed spina bifida the cranium was unremarkable. When the data were available, levels of amniotic fluid alpha-fetoprotein were always abnormally elevated with open spina bifida and within normal limits with closed forms. Conclusion In this study 7% of cases of spina bifida diagnosed in utero were closed. The differentiation between open and closed forms is best shown by the sonographic demonstration of abnormal or normal cranial anatomy. [ABSTRACT FROM AUTHOR]