OBJECTIVES: Emerging evidence suggests that diabetes may increase the risk of cancers. Available evidence on prostate cancer, however, is conflicting. We therefore examined the association between type 2 diabetes and risk of prostate cancer by conducting a detailed meta-analysis of prospective cohort studies published regarding this subject. METHODS: PubMed database, CINAHL, EMBASE, and bibliographies of retrieved articles were searched for prospective cohort studies (published between 1970 and 2011) investigating the relationship between type 2 diabetes and prostate cancer. Pooled risk ratio (RR) was calculated using random-effects model (DerSimonian and Laird method). Heterogeniety was calculated using Cochrane's Q test. To find out heterogeniety we performed subgroup analysis. Sensitivity analysis and cumulative meta-analysis were also done. RESULTS: Twenty-nine cohort studies involving 5.1 million participants and 92,331 prostate cancer cases detected a significant inverse association between type 2 diabetes and risk of prostate cancer (RR 0.87 (95% CI 0.80-0.94)). Sensitivity analysis done by excluding one outlier further strengthened our negative association (RR 0.82, 95% CI 0.76-0.84). This association was not substantially modified by Prostate-specific antigen testing, number of covariates adjustment and adjustment for BMI or obesity. Cumulative meta-analysis showed a change in trend of reporting risk of prostate cancer from positive to negative in type 2 diabetes patients between 1970 and 2011. No evidence of publication bias was observed. CONCLUSIONS: Our meta-analysis provides strongest evidence supporting that type 2 diabetes is significantly inversely associated with risk of developing prostate cancer.