Mesangial expansion caused by cell proliferation and glomerular extracellular matrix accumulation is one of the earliest renal abnormalties observed at the onset of hyperglycemia in diabetes mellitus. Transcription factor Sp1 is implicated in the transcriptional regulation of a wide range of genes participating in cell proliferation, and is assumed to play an essential role in mesangial expansion, transforming growth factor (TGF)-$eta$1, plasminogen activator inhibitor (PAI)-1. We have generated a phosphorothioated double-stranded Sp1-decoy oligodeoxynucleotide that effectively blocks Sp1 binding to the promoter region for transcriptional regulation of TGF-$eta$1 and PAI-1. The Sp1 decoy oligodeoxynucleotide suppressed transcription of these cytokines and proliferation of primary rat mesangial cells in response to serum stimulation. These results suggest that the Sp1 decoy oligodeoxynucleotide could bea powerful tool in preventing the pathogenesis of renal hypertrophy.