Some question whether tocolytic drugs reduce uterine activity and prolong gestation. The interval from discontinuance of tocolytics until spontaneous labor and delivery in patients (n = 69) with documented preterm labor (PTL) versus subjects receiving prophylactic tocolytic therapy (n = 41) was studied. Women with documented PTL delivered sooner after cessation of tocolytics (6.1 +/- 6.9 days) than control (C) patients (14.7 +/- 10.8 days, P less than 0.001). Also, 28 of the 69 (41%) patients in the PTL group delivered within 24 h of discontinuation of tocolysis compared to 4 (10%) in the C group (P less than 0.0004). We conclude that tocolytic therapy for documented preterm labor suppresses uterine activity and when these agents are discontinued, contractions return and labor ensues.