PURPOSE: To describe the distribution of retinal vascular calibers and their associations with ethnicity, gender, and birth parameters in children. METHODS: This was a school-based cross-sectional study of 768 children aged 7 to 9 years who participated in the Singapore Cohort Study of the Risk Factors for Myopia (SCORM). Participants had retinal photographs taken in 2001. Retinal vascular calibers were measured with computer-based program and summarized as average caliber of arterioles and venules in that eye. Associations of retinal vascular caliber with ethnicity, gender, and various birth factors were analyzed. RESULTS: In this population, the mean retinal arteriolar caliber was 156.4 microm (95% confidence interval [CI], 155.4-157.3) and venular caliber was 225.4 microm (95% CI, 224.1-226.8). The retinal arteriolar caliber was significantly narrower in Chinese (154.9 microm), compared with Malay (158.6 microm) and Indian (158.5 microm) children. Retinal venular caliber was also narrower in Chinese (223.3 microm) compared with Malay (230.8 microm) and Indian (229.0 microm) children. These differences were statistically significant, even after adjustments for age, gender, family income, parental education, body mass index, height, birth weight, axial length, and spherical equivalent (P = 0.05 for arteriolar caliber; P = 0.002 for venular caliber). In multivariate analysis, there were no significant gender differences in retinal vascular caliber. Birth factors, including birth weight, birth length, head circumference, and gestational age, were not significantly associated with changes in either retinal arteriolar or venular caliber. CONCLUSIONS: The results show ethnic variation in retinal vascular caliber in Singaporean children. No association of birth parameters with retinal vascular caliber was found. Because retinal vascular caliber is related to various cardiovascular and ocular diseases, it is possible that ethnic variations in retinal vascular caliber should be taken into consideration in future studies.