It has been suggested that diets which enhance diurnal insulin secretion, such as a high glycemic index (GI) and glycemic load (GL) diet, can be expected to increase homocysteine levels. We investigated the hypothesis that dietary GI and GL are positively associated with serum homocysteine concentration in a group of free-living young Japanese women. This preliminary cross-sectional study included 1050 female Japanese dietetic students aged 18 to 22 years. Dietary intake was assessed using a validated, self-administered, comprehensive diet history questionnaire. Fasting blood samples were collected and serum homocysteine concentrations were measured. Adjustment was made for survey year, region, municipality level, current smoking, current alcohol consumption, dietary supplement use, physical activity, body mass index, energy intake, and intakes of B vitamins (folate, vitamin B6, vitamin B12, and riboflavin). After adjustment for nondietary confounding factors, both dietary GI and GL were positively associated with homocysteine concentration (both P for trend=.001). The positive association between dietary GI and homocysteine concentration remained after further adjustment for intakes of B vitamins. Mean (95% confidence interval) values of serum homocysteine concentration for each quintile of dietary GI were 6.9 (6.7-7.2), 7.1 (6.8-7.3), 7.0 (6.7-7.2), 7.4 (7.2-7.7), and 7.3 (7.0-7.6) μmol/L, respectively (P for trend = .04). Conversely, there was no association between dietary GL and homocysteine concentration after further adjustment for intakes of B vitamins (P for trend = .40). To conclude, in a group of free-living young Japanese women, dietary GI, but not GL, was independently and positively associated with serum homocysteine concentration.