To date, the relationship between obesity and the incidence of lung cancer remains unclear and inconclusive. Thus, we conducted a meta-analysis of published studies to provide a quantitative evaluation of this association. Relevant studies were identified through PubMed and EMBASE databases from 1966 to December 2011, as well as through the reference lists of retrieved articles. A total of 31 articles were included in this meta-analysis. Overall, excess body weight (body mass index, BMI ≥ 25 kg/m2) was inversely associated with lung cancer incidence (relative risk, RR = 0.79; 95% confidence interval, CI: 0.73–0.85) compared with normal weight (BMI = 18.5-24.9 kg/m2). The association did not change with stratification by sex, study population, study design, and BMI measurement method. However, when stratified by smoking status, the inverse association between excess body weight and lung cancer incidence in current (RR = 0.63, 95% CI: 0.57–0.70) and former (RR = 0.73, 95% CI: 0.58–0.91) smokers was strengthened. In non-smokers, the association was also statistically significant (RR = 0.83, 95% CI: 0.70–0.98), although the link was weakened to some extent. The stratified analyses also showed that excess body weight was inversely associated with squamous cell carcinoma (RR = 0.68, 95% CI: 0.58–0.80) and adenocarcinoma (RR = 0.79, 95% CI: 0.65–0.96). No statistically significant link was found between excess body weight and small cell carcinoma (RR = 0.99, 95% CI: 0.66–1.48). The results of this meta-analysis indicate that overweight and obesity are protective factors against lung cancer, especially in current and former smokers.