Progressive nephropathies are characterized by the enhanced accumulation of extracellular matrix in the kidney. Overproduction of transforming growth factor-beta (TGF-beta) was shown to result in pathological tissue fibrosis through the accumulation of extracellular matrix proteins. It has been proposed that angiotensin II stimulates TGF-beta production. Despite accumulating data supporting the effects of angiotensin-converting enzyme (ACE) inhibitors on the attenuation of TGF-beta in vitro and in rats, such studies in humans are lacking. The present study sought to determine the effects of ACE inhibitors on TGF-beta1 in patients with glomerulonephritis. Using competitive polymerase chain reaction and the sandwich enzyme-linked immunosorbent assay, TGF-beta1 messenger RNA (mRNA) abundance and TGF-beta1 protein levels were measured. Patients with immunoglobulin A nephropathy administered ACE inhibitors showed significantly lower renal TGF-beta1 gene expression than patients not administered these medications (mean ratios of TGF-beta1/beta-actin, 4.27 +/- 0.62 [SEM] versus 14.81 +/- 3.87; P < 0.05), whereas no difference was noted between patients administered ACE inhibitors and healthy controls (4.27 +/- 0.62 versus 2.78 +/- 0.71). ACE inhibitor therapy did not affect TGF-beta1 mRNA expression in freshly isolated mononuclear cells. Urine and serum TGF-beta1 protein levels were not affected by the administration of ACE inhibitors. However, possibly a longer duration of treatment would decrease TGF-beta1 levels in urine or blood. In conclusion, we observed a significant reduction in TGF-beta1 expression in the kidney by ACE inhibitors, and this suggests that the effects of ACE inhibitors observed in animals can be extrapolated to patients with chronic renal disease.