Connective tissue growth factor (CTGF) is overexpressed in diabetic nephropathy (DN) and has therefore been implicated in its pathogenesis. The objective of the present study was to determine the tissue distribution of increased CTGF expression and the relationship of plasma, urinary, and renal CTGF levels to the development and severity of DN. We studied the relationship between CTGF and renal pathology in streptozotocin (STZ)-induced diabetes in C57BL/6J mice. Diabetic and age-matched control mice were killed after 1, 2, 4, and 9 wk of diabetes. In addition, key parameters of diabetes and DN were analyzed in 10-mo-old diabetic ob/ob mice and their ob/+ littermates. STZ-induced diabetic mice showed a significantly increased urinary albumin excretion after 1 wk and increased mesangial matrix score after 2 wk. Increased renal fibronectin, fibronectin ED-A, and collagen IVα₁ expression, as well as elevated plasma creatinine levels, were observed after 9 wk. After 2 wk, CTGF mRNA was upregulated threefold in the renal cortex. By 9 wk, CTGF mRNA was also increased in the heart and liver. In contrast, transforming growth factor-β₁ mRNA content was significantly increased only in the kidney by 9 wk. Renal CTGF expression was mainly localized in podocytes and parietal glomerular epithelial cells, and less prominent in mesangial cells. In addition, plasma CTGF levels and urinary CTGF excretion were increased in diabetic mice. Moreover, albuminuria strongly correlated with urinary CTGF excretion (R = 0.83, P < 0.0001). Increased CTGF expression was also demonstrated in type 2 diabetic ob/ob mice, which points to a causal relationship between diabetes and CTGF and thus argues against a role of STZ in this process. The observed relationship of podocyte and urinary CTGF to markers of DN suggests a pathogenic role of CTGF in the development of DN.