Uterine contractility patterns were studied in an attempt to identify those women on tocolytic therapy at risk for preterm delivery. One hundred sixteen women treated successfully after an episode of preterm labor were followed with an ambulatory tocodynamometer system. Contractions were monitored twice daily for 1 hour until delivery or until 36 completed weeks of gestation. In this group, 52 women (45%) delivered preterm and 64 (55%) delivered at term. The analysis revealed that 1) from 24-29 weeks' gestation, there was no significant difference in the uterine activity between the two groups; and 2) beginning at 30 weeks' gestation and continuing until the end of the 36th week of gestation, uterine activity was significantly greater in women destined to deliver preterm. These observations suggest that the patterns of daily uterine contractility observed in patients after an episode of preterm labor can identify those at greater risk for a preterm delivery.