We aimed to determine whether retinal microvascular changes in vessel calibers at baseline are associated with the future risk of developing the metabolic syndrome over 4 years in an adult cohort of Japanese individuals (n=90) who attended a health-screening program. Retinal vessel caliber was calculated as the central retinal artery equivalent and vein equivalent (CRAE) from non-mydriatic digital fundus images using semiautomated standardized software. There were 18 cases (20%) that developed metabolic syndrome over 4 years. Narrower arteriolar caliber at baseline was associated with an increased risk of incident metabolic syndrome over 4 years after adjusting for potential confounding risk factors including individual cardiovascular risk factors related to the metabolic syndrome (adjusted odds ratio per 1 s.d. change in CRAE: 2.92, 95% confidence interval 1.03, 8.24; P=0.043). Persons with wider venular caliber at baseline were more likely to have incident metabolic syndrome, but this was not statistically significant. Retinal vascular caliber might provide independent and useful information to predict incident metabolic syndrome in a health screening program.