Background: It is suggested that excluding types of study in Cochrane reviews based on a rigid hierarchy of evidence could result in empty or inconclusive reviews, thereby depriving researchers and policy makers of the ‘best available’ evidence to guide policy and practice. Objectives: To evaluate the relationship between restricting types of studies to randomized controlled trials (RCTs) and the proportion of inconclusive Cochrane reviews of interventions addressing the Millennium Development Goals and of a non-communicable health condition. Methods: Full text of all reviews in Issue 1, 2009 of The Cochrane Library from the Cochrane Pregnancy and Childbirth, Neonatal, Acute Respiratory Infections, HIV/AIDS, STD, Infectious Diseases and Heart groups were checked by two reviewers for the proportions with informative conclusions (interventions were likely to be beneficial, harmful, trade-off between benefits and harms) and those that had insufficient data, too few or no included studies to make conclusions. We estimated the odds (with 95% confidence intervals) of restricting types of studies to RCTs versus including other study types; the numbers of trials and participants included resulting in inconclusive and empty reviews versus informative reviews using binary logistic regression. Results: Of 851 reviews, 370 (44%) were inconclusive, of which 72 were empty reviews (9% of total; 20% of inconclusive reviews). Restricting study type to RCT did not significantly increase the odds of inconclusive (OR 1.0; 95% CI 0.7 to 1.3) or empty reviews (OR 0.8; 95% CI 0.5 to 1.3). The odds of resulting in inconclusive reviews on logistic regression increased significantly for reviews (with less than compared to greater than 1000 participants OR 2.6; 95% CI 1.9 to 3.6). Conclusions: Restricting types of studies to RCTs to minimize bias did not result in ‘empty’ or inconclusive reviews in this sample of intervention reviews of priority health conditions.