We determined the feasibility of prenatal prediction of type 1 homozygous alpha-thalassemia at 12 to 13 weeks of gestation by either abdominal or vaginal (or both) ultrasonographic examination to measure the fetal cardiothoracic ratio in 135 at-risk pregnancies. Forty-three fetuses were affected by homozygous type 1 alpha-thalassemia. The mean cardiothoracic ratio was significantly larger than that of the unaffected fetuses (0.54 vs 0.45, P <.0005), with no overlap between the 2 groups. A cardiothoracic ratio cutoff point of >/=0.5 was 100% sensitive and specific for disease.