It has been documented that green tea (GT) and its catechin components improve renal failure and inhibit the growth of mesangial cells. In the present study we examined the long-term effect of GT extract on streptozotocin (STZ)-induced diabetic nephropathy and on the glycogen accumulation in the kidney tubules. Male Sprague–Dawley rats were randomly assigned to normal control groups (2, 6, 8 and 12 weeks) and five diabetic groups (n 10) of comparable age. A GT diabetic group received 16 % concentration of GT for 12 weeks post-diabetes induction as their sole source of drinking water. GT treatment significantly (P < 0·01) reduced the serum glucose, glycosylated protein, serum creatinine and blood urea N levels by 29·6 (sem 3·7), 22·7 (sem 5·2), 38·9 (sem 10) and 41·7 (sem 1·9) %, respectively, compared with the diabetic group of comparable age. In addition, the GT-treated group showed a significant 44 (sem 10·8) % higher creatinine clearance (Ccr) compared with the untreated diabetic group. Likewise, GT reduced the urea N, creatinine, glucose and protein excretion rates by 30 (sem 7·6), 35·4 (sem 5·3), 34·0 (sem 5·3) and 46·0 (sem 13·0) % compared with the 12 weeks diabetic group. Administration of GT to 12 weeks diabetic rats significantly (P < 0·001) prevented (99·98 (sem 0·27) % less) the accumulation of glycogen in the kidney tubules. These results indicate that in STZ diabetes, kidney function appears to be improved with GT consumption which also prevents glycogen accumulation in the renal tubules, probably by lowering blood levels of glucose. Therefore, GT could be beneficial additional therapy in the management of diabetic nephropathy.