Objective: To study whether knowledge of cervical length (CL) is useful in reducing the length of hospital stay in women admitted because of threatened preterm labor. Methods: We performed a single-center, parallel, randomized trial at the Hospital Clínic of Barcelona. Inclusion criteria were single pregnancy, gestational age (GA) between 24⁺⁰ and 35⁺⁶ weeks, Bishop score <6, no parturition within 24-48 h after admission, and no clinical signs of chorioamnionitis, vaginal bleeding, or nonreassuring fetal status. CL measurement was performed 24-48 h after admission. In the control group, the patient and the physician in charge were blinded. In the study group, this information was given; if CL was >25 mm, early discharge within 12-24 h from randomization was recommended. Length of hospital stay was the main outcome. Results: After randomization, 149 patients had complete follow-up (control group, n = 74; study group, n = 75). The mean (SD) length of stay was significantly shorter - 3.0 (2.2) vs. 4.0 (2.0) days (p = 0.004) - in the study group, with a higher proportion of women remaining hospitalized ≤3 days (relative risk [95% confidence interval] 0.43 [0.26-0.70]), with no differences in GA at delivery or preterm birth rate. Conclusions: Knowledge of CL in women admitted because of threatened preterm labor is useful in reducing length of stay, with no impact on GA at delivery or preterm birth rate.