To study the effects of the intake of green tea and polyphenols, which are a component of green tea, on insulin resistance and systemic inflammation, a randomized controlled trial was conducted on 66 patients aged 32-73 y (53 males and 13 females) with borderline diabetes or diabetes. Subjects in the intervention group were asked to take a packet of green tea extracts/powder containing 544 mg polyphenols (456 mg catechins) daily, which was a dose that could be taken without difficulty, and were asked to divide the green tea extracts/powder in a packet into 3 or 4 fractions dissolved in hot water everyday and to take a fraction after every meal or snack for 2 mo, in addition to daily food intake. The subjects in the control group were simply followed. To calculate the level of green tea polyphenol intake that the subject usually drank at home, the subject was asked to taste 3 teas of different strengths (1, 2 and 3%) and the tea that was closest to the one that the subject drank at home, was selected by each subject. After 2 mo, the mean daily polyphenol intake in the intervention group was 747 mg, which was significantly higher than that of 469 mg in the control group. In the intervention group, the body weight, BMI, systolic and diastolic blood pressures, blood glucose level, Hb A1c level, insulin level and HOMA index after taking the supplementation for 2 mo, were lower than the respective value before intervention: however, these parameters in the intervention group at 2 mo did not significantly differ from those in the control group. Within the intervention group, changes in insulin level tended to be associated with changes in polyphenol intake. In addition, changes in BMI were associated with changes in blood glucose level and insulin level. In conclusion, the daily supplementary intake of 500 mg green tea polyphenols did not have clear effects on blood glucose level, Hb A1c level, insulin resistance or inflammation markers. The positive correlation between the level of polyphenol intake and insulin level warrants further studies on the effect of green tea on insulin resistance.