A total of 4517 successful antepartum nonstress tests (NST) was performed on 2003 high- (28%) and low- (72%) risk obstetric patients. Most patients were monitored from 32 to 34 weeks' gestation onward. A reactive NST was defined as 2 accelerations in 10 minutes--15 beats/min minimum amplitude; 15 seconds minimum duration. The test was repeated at the next visit in low-risk patients with reactive NST or in 1 week in high-risk patients with this response. Nonreactive tests were followed immediately by a stress test in high-risk patients, but only after repeat nonreactive NST within 24 hours in low-risk patients. About 88% of low-risk and 86% of high-risk patients demonstrated ractive NST only. Late decelerations during subsequent stress testing or labor, low Apgar scores, and perinatal deaths were more common in low-risk pregnancies than in high-risk pregnancies and more common in those with nonreactive NST than in those with reactive NST. High-risk/reactive NST babies, however, fared better than low-risk/nonreactive NST babies. Of the 16 perinatal deaths 6 died antepartum, 2 died in labor, and 8 died as neonates. Presumed asphyxia accounted for 6 deaths while significant anomaly accounted for 5; sepsis and trauma accounted for 2 each. In only 2 instances was a reactive NST followed by a death from apparent asphyxia. Routine NST testing appears to improve the resolution of maternal risk classification and may contribute to better perinatal outcome.