Inconsistent results regarding the association between abdominal obesity and the risk of colorectal adenoma (CRA), the precursor of colorectal cancer (CRC), have been reported. To provide a quantitative assessment of this relationship, we summarized the evidence from observational studies in categorical, linear dose-response meta-analyses. We searched MEDLINE and EMBASE for studies of waist circumference (WC) and/or waist-hip ratio (WHR) and CRA risk published until the end of October 2011. Summary relative risks (SRRs) with 95% confidence intervals (CIs) were summarized using a random-effects model. Between-study heterogeneity was assessed using the Cochran's Q and I statistics. A total of 21 studies (four case-control studies, 12 cross-sectional studies, and five cohort studies) were included in this meta-analysis. Overall, the SRRs of CRA were 1.39 (95% CI: 1.24-1.56) for the highest versus the lowest level of WC and 1.22 (95% CI: 1.10-1.35) for WHR (P-value for heterogeneity 0.013 and 0.458, respectively). In linear dose-response analysis, a 10-cm increase in WC was related to an increased risk of CRA (SRR, 1.19; 95% CI, 1.13-1.26) and a 0.1-unit increment in WHR gave 1.16 (95% CI: 1.06-1.26). Subgroup analyses revealed that the increased risk of CRA in abdominally obese individuals was independent of geographic location, design, sex, and confounders: alcohol use, smoking status, and family history of colorectal cancer. However, BMI may be a confounder for the association between WC and CRA risk. These results suggest that abdominally obese individuals, both men and women, may have an increased risk of CRA.