Among 12,572 pregnant women referred for ultrasound examination from 1985-1990, 76 fetuses had ultrasonographic findings of hydrops fetalis, ten immune and 66 nonimmune. Fetuses with cystic hygroma (20), heart defects or arrhythmias (13), or other congenital anomalies (15) accounted for the majority of the nonimmune cases. Antenatal chromosomal studies were available in 42 fetuses with nonimmune hydrops, of which 14 (34%) were abnormal with seven monosomes and six trisomies. Seventeen cases of hydrops (22%) were classified as idiopathic because they had no recognizable etiology. It is concluded that: 1) The ultrasonographic incidence of fetal hydrops in referral centers can be as high as one in 165 pregnancies; 2) most cases of fetal hydrops are of the nonimmune type, which can occur in a low-risk population and can be detected with early second-trimester ultrasound screening; and 3) the complexity of this condition and the high rate of chromosomal abnormalities require referral to a high-risk center for evaluation and pregnancy management.