Twenty-seven cases of alobar/semilobar holoprosencephaly were reviewed to determine and classify the associated facial abnormalities detected with prenatal sonography. All but one case were diagnosed prospectively with sonography. Facial abnormalities were present in 24 of 27 cases and were detected in 14 (58%) of 24 fetuses on prenatal sonography. Prenatal abnormalities detected by sonography included cyclopia (four of five), ethmocephaly (two of three), cebocephaly (one of three), midline cleft lip (four of eight), lateral cleft lip (two of two), and mild hypotelorism (one of three). One or more extrafacial anomalies were present in 14 (52%) of 27 fetuses. Other major structural anomalies detected prenatally by sonography included meningomyelocele (two of two), renal dysplasia (five of six), omphalocele (three of four), esophageal atresia (zero of three), and cardiac defects (one of seven). Chromosomal analysis revealed abnormal karyotype in 13 (50%) of the 26 fetuses in which it was performed; the most common abnormality was trisomy 13 (seven cases). The perinatal mortality rate was 89% (24 of 27); three neonates with holoprosencephaly were alive when discharged from the hospital. It is concluded that when a major cystic abnormality of the fetal brain is detected, prenatal sonographic analysis of midline facial defect may allow more definitive diagnosis of alobar or semilobar holoprosencephaly; affected fetuses often have other major structural abnormalities, and the outcome is nearly always fatal.