Purpose To investigate the prevalence of underpowered randomized controlled trials (RCTs) presented at American Society of Clinical Oncology (ASCO) annual meetings. Methods We surveyed all two-arm phase III RCTs presented at ASCO annual meetings from 1995 to 2003 for which negative results were obtained. Post hoc calculations were performed using a power of 80% and an α level of .05 (two sided) to determine sample sizes required to detect small, medium, and large effect sizes. For studies reporting a proportion or time-to-event as primary end point, effect size was expressed as an odds ratio (OR) or hazard ratio (HR), respectively, with a small effect size defined as OR/HR ≥ 1.3, medium effect size defined as OR/HR ≥ 1.5, and large effect size defined as OR/HR ≥ 2.0. Logistic regression was used to identify factors associated with lack of statistical power. Results Of 423 negative RCTs for which post hoc sample size calculations could be performed, 45 (10.6%), 138 (32.6%), and 233 (55.1%) had adequate sample size to detect small, medium, and large effect sizes, respectively. Only 35 negative RCTs (7.1%) reported a reason for inadequate sample size. In a multivariable model, studies that were presented at oral sessions (P = .0038), multicenter studies supported by a cooperative group (P < .0001), and studies with time to event as primary outcome (P < .0001) were more likely to have adequate sample size. Conclusion More than half of negative RCTs presented at ASCO annual meetings do not have an adequate sample to detect a medium-size treatment effect.