Emerging epidemiological evidence suggests that higher coffee consumption may reduce the risk of type 2 diabetes. To examine the association between habitual coffee consumption and risk of type 2 diabetes and related outcomes. We searched MEDLINE through January 2005 and examined the reference lists of the retrieved articles. Because this review focuses on studies of habitual coffee consumption and risk of type 2 diabetes, we excluded studies of type 1 diabetes, animal studies, and studies of short-term exposure to coffee or caffeine, leaving 15 epidemiological studies (cohort or cross-sectional). Information on study design, participant characteristics, measurement of coffee consumption and outcomes, adjustment for potential confounders, and estimates of associations was abstracted independently by 2 investigators. We identified 9 cohort studies of coffee consumption and risk of type 2 diabetes, including 193 473 participants and 8394 incident cases of type 2 diabetes, and calculated summary relative risks (RRs) using a random-effects model. The RR of type 2 diabetes was 0.65 (95% confidence interval [CI], 0.54-0.78) for the highest (>or=6 or >or=7 cups per day) and 0.72 (95% CI, 0.62-0.83) for the second highest (4-6 cups per day) category of coffee consumption compared with the lowest consumption category (0 or