We investigated prospectively the association between serum levels of interleukin (IL)-18 and the risk of type 2 diabetes in a case-cohort study conducted in middle-aged men and women who represented 7,936 participants of the three MONItoring of trends and determinants in CArdiovascular disease (MONICA)/Cooperative Research in the Region of Augsburg (KORA) surveys. Levels of IL-18 were measured in stored samples of 527 case subjects with incident type 2 diabetes and 1,698 noncase subjects. Elevated levels of IL-18 were associated with a significantly increased risk of type 2 diabetes after adjustment for age, sex, survey, BMI, systolic blood pressure, ratio of total cholesterol to HDL cholesterol, physical activity, alcohol intake, smoking status, and parental history of diabetes. Hazard ratios and 95% confidence intervals comparing quartile extremes were 1.73 (1.25-2.40). Further adjustment for C-reactive protein and IL-6 had no impact on the observed associations. However, the risk of developing type 2 diabetes was highest among subjects with elevated levels of both IL-18 and CRP or IL-18 and IL-6, respectively. In conclusion, elevated levels of IL-18 are associated with a considerably increased risk of type 2 diabetes. This association is independent of a generalized proinflammatory state, but subjects with elevated levels of several inflammatory markers seem to be particularly prone to develop type 2 diabetes.