Objectives: To investigate the relative values in the prediction of type 2 diabetes and coronary heart disease (CHD) by the metabolic syndrome (MS) as defined by the ATPIII and by its modification of the Turkish Adult Risk Factor Study (TEKHARF-def) and selection of most appropriate definition. Materials and Methods: Prospective evaluation of 1683 men and 1718 women, aged ≥28 years participating in the TEKHARF study surveys 1997/98 and 2002/03 with a mean follow-up of 5.9 years. The modification involved especially abdominal obesity (≥95 cm in men, ≥91 cm in women). Results: After exclusion of participants with diabetes at baseline and adjustment for sex and age, both MS definitions predicted the development of diabetes with virtually identical relative risks (RR) (ATPIII 2.85 [95%CI 2.14; 3.80]; TEKHARF 2.84 [95%CI 2.13; 3.81]. After similar exclusion and adjustments, both MS definitions predicted significantly the development of CHD with similar RRs (ATPIII 2.10 [95%CI 1.64; 2.68] in 36% of the cohort; TEKHARF-def 1.90 [95%CI 1.49; 2.43] in 39.6% of the cohort. For both outcomes, the TEKHARF-def provided higher predictive values in men, and (because of the high density lipoprotein (HDL)-cholesterol cutoff) the ATPIII definition in women. Absolute annual CHD risk in individuals with MS exceeded on average 2%, while age ≥50 years constituted the most appropriate indicator of further elevated risk in both genders. Most suitable modifications of the ATPIII definition are proved to be impaired fasting glucose (IFG) ≥100 mg/dl and in men ≥95 cm of waist circumference. Most CHD cases afflicting Turkish adults (namely 61% in men and 69% in women) originated from the latter definition of MS. Conclusions: In predicting diabetes and CHD risk, the TEKHARF-def MS is more valuable in men; the ATPIII definition modified for IFG (≥100 mg/dl) should be adopted in women. In 2 out of every 3 cases, CHD originates from MS among Turks, and age ≥50 years is a good indicator of higher risk in both genders.