Objectives: To study the statistical power of randomized clinical trials and examine developments over time. Study Design and Setting: We analyzed the statistical power in 136,212 clinical trials between 1975 and 2014 extracted from meta-analyses from the Cochrane database of systematic reviews. We determined study power to detect standardized effect sizes, where power was based on the meta-analyzed effect size. Average power, effect size, and temporal patterns were examined for all meta-analyses and a subset of significant meta-analyses. Results: The number of trials with power ≥80% was low (7%) but increased over time: from 5% in 1975–1979 to 9% in 2010–2014. In significant meta-analyses, the proportion of trials with sufficient power increased from 9% to 15% in these years (median power increased from 16% to 23%). This increase was mainly due to increasing sample sizes, while effect sizes remained stable with a median Cohen's h of 0.09 (interquartile range 0.04–0.22) and a median Cohen's d of 0.20 (0.11–0.40). Conclusion: This study demonstrates that sufficient power in clinical trials is still problematic, although the situation is slowly improving. Our data encourage further efforts to increase statistical power in clinical trials to guarantee rigorous and reproducible evidence-based medicine.