Acardiac twinning affects 1 in 100 monozygotic twin pregnancies and 1 in 35,000 pregnancies overall. The presence of an acadiac twin requires the normal (or "pump") twin to provide circulation for itself, as well as the acardiac sibling. In many cases the acardiac twin is almost equal to the normal twin. The principal perinatal problems associated with acardiac twinning are pump-twin congestive heart failure, maternal hydramnios, and preterm delivery. We reviewed the perinatal courses of 49 acardiac twin pregnancies to identify factors prognostic of favorable outcome. The overall perinatal mortality was 55%, primarily associated with prematurity. Mean (+/- SD) gestational age at delivery was 29 +/- 7.3 weeks, with a mean (+/- SD) normal twin weight of 1378 +/- 1047 gm. The acardiac weight averaged 651 +/- 571 gm. However, the occurrence of hydramnios, the occurrence of preterm labor, and perinatal outcome were strongly related to the ratio of the acardiac and pump-twin's weight. The mean overall ratio of the twin weights was 52% +/- 42%. However, the mean weight ratio for patients delivered before 34 weeks was 60% versus 29% (p less than 0.04). Preterm delivery was strongly associated with the development of hydramnios and congestive heart failure in the pump twin (p less than 0.01). If the twin-weight ratio was above 70% (25% of cases), the incidence of preterm delivery was 90%; hydramnios, 40%; and pump-twin congestive heart failure, 30% compared with 75%, 30%, and 10%, respectively, when the ratio was less (p less than 0.05). Regression of the weight of the acardiac twin against its longest dimension (L) resulted in this equation for prediction of acardiac weight: Weight (grams) = 1.2 L2-1.7L; r = 0.79, p less than 0.01. These data suggest that estimation of the relative weights in acardiac twins provides prognostic information regarding outcome. Poor outcome occurs with congestive heart failure and hydramnios in the normal twin. Use of the above data may assist in counseling patients and determining optimal management.