Background: Tocolytic therapy to stop preterm labor is an important intervention in obstetrics. Despite themany randomized controlled trials (RCTs) of different interventions, no comprehensive updated summary evaluation of the evidence has been undertaken. Objective: To assess the quality of tocolysis RCTs and to determine trial factors contributing to better quality evidence. Methods: The Cochrane Central Register of Controlled Trials, MEDLINE, MEDLINE In-Process, EMBASE, and CINAHL were searched for terms preterm labor , tocolytic , or obstetric labor, premature up to August 1, 2009. Data regarding study design, characteristics, number of participants, and outcomes reported were extracted by at least 2 review authors. Study quality was assigned utilizing the Cochrane Handbook methodology and categories. Trends for quality over time, the impact of study size, and the individual drugs compared were analyzed for impact on overall quality of trials. Results: Of the 3,197 titles initially identified, 89 RCTs of tocolytic therapy were reviewed. Of the 6 quality areas, 10 (11.2%) trials satisfied all areas, while only 1 trial (1.1%) met 1 area. The mean number of adequate areas was 4.1 1.2. Overall, 52 (58.4%) of the trials achieved high quality categorization. The total number of subjects was weakly correlated with the number of adequate quality areas (p=0.007). The mean number of quality areas was higher for placebo-containing trials (4.7 vs. 3.9, p=0.009) and lower for calcium channel inhibitor-containing trials (3.6 vs. 4.3, p=0.03). More recent trials demonstrated higher quality (p=0.036). Compared to studies performed in Asia (37.5%), those in North America (71.8%) and Europe (57.1%) were rated as high quality studies more frequently (p=0.03). Controlling for multiple trial factors, the location and decade were significant predictors of overall trial quality. Conclusions: The majority of tocolysis RCTs are of high quality. Larger trials, more recent trials, and placebo-controlled trials were associated with higher quality scores.