We investigated the effects of continuous ingestion of a catechin-rich beverage in patients with type 2 diabetes who were not receiving insulin (Ins) therapy in a double-blind controlled study. The participants ingested green tea containing either 582.8 mg of catechins (catechin group; n = 23) or 96.3 mg of catechins (control group; n = 20) per day for 12 weeks. At week 12, the decrease in waist circumference was significantly greater in the catechin group than in the control group. Adiponectin, which is negatively correlated with visceral adiposity, increased significantly only in the catechin group. Although the increase in Ins at week 12 was significantly greater in the catechin group than in the control group, no apparent difference was noted between the two groups in glucose and hemoglobin A(1c). In patients treated with insulinotropic agents, the increase in Ins at week 12 was significantly greater in the catechin group than in the control group. This significant increase in Ins levels was observed only in the catechin group. In the catechin group receiving other treatments, Ins levels remained unchanged. In addition, in patients treated with insulinotropic agents, the decrease in hemoglobin A(1c) at week 12 was significantly greater in the catechin group than in the control group. These results suggest that a catechin-rich beverage might have several therapeutic uses: in the prevention of obesity; in the recovery of Ins-secretory ability; and, as a way to maintain low hemoglobin A(1c) levels in type 2 diabetic patients who do not yet require Ins therapy.