Renal mesangial cell hypertrophy is a characteristic of diabetic nephropathy as well as a response to renal stress or injury. Because hypertrophy is a result of increased protein content per cell without DNA replication, those proteins that control the cell cycle, such as the cyclin kinase inhibitor p21, represent fertile ground for studying the mechanism of this structural alteration. A key role for p21 in promoting mesangial cell (MC) hypertrophy has been established using p21 knockout mouse models. Furthermore, some of the biologic effects of IGF-1, including cell proliferation, have been shown to be positively influenced by p21. In an attempt to begin to translate these findings ultimately to the bedside, methods to attenuate p21 levels in wild-type kidney cells were examined. With the use of a phosphorothioated antisense oligodeoxynucleotide (ODN) to p21, which has previously been shown to decrease specifically and effectively p21 protein levels in a variety of cell types, it is shown that attenuation of p21 in MC leads to a dose-dependent reduction of hypertrophy in the milieu of hyperglycemic culture media. Furthermore, the hypertrophic effect of the IGF-1 on MC is also attenuated using the same antisense p21 ODN. There was no evidence of apoptosis or other toxicity in MC transfected with the concentrations of antisense p21 ODN used in these experiments. Because the use of antisense ODN in human disease is already established in other medical disciplines, the stage is now set for the use of antisense p21 ODN to attenuate renal cell hypertrophy in vivo, leading to a new strategy for treatment of diabetic nephropathy and other diseases characterized by MC hypertrophy.