Recent evidence suggests that tea from Camellia sinensis (eg, green, oolong, and black tea) may have a hypoglycemic effect. We evaluated the ability of an extract of green and black tea to improve glucose control over a 3-month period. A double-blind, placebo-controlled, randomized multiple-dose (0, 375, or 750 mg per day for 3 months) study in adults with type 2 diabetes mellitus not taking insulin was performed. The primary end point was change in glycosylated hemoglobin at 3 months. The 49 subjects who completed this study were predominantly white with an average age of 65 years and a median duration of diabetes of 6 years, and 80% of them reported using hypoglycemic medication. After 3 months, the mean changes in glycosylated hemoglobin were +0.4 (95% confidence interval, 0.2-0.6), +0.3 (0.1-0.5), and +0.5 (0.1-0.9) in the placebo, 375-mg, and 750-mg arms, respectively. The changes were not significantly different between study arms. We did not find a hypoglycemic effect of extract of green and black tea in adults with type 2 diabetes mellitus.