Bisulfite modification of cytosine residues is a widely used method for analyzing genomic DNA methylation. Despite its robustness, degradation of DNA during modification has hampered the application to the analysis of small amounts of DNA. We show that the addition of large volume bisulfite-treated heterogeneous DNA promotes the amplification of the targeted region in bisulfite-treated DNA by PCR. The addition of untreated DNA did not promote the amplification. The addition of large volume bisulfite-treated heterogeneous DNA neither promoted the amplification when untreated DNA was used as template. PCR products were detected when a ten thousandth of aliquot of bisulfite-treated 1 μg of human genomic DNA (0.1 ng) was used as template. This figure is equivalent to that of human genomic DNA derived from as little as 15 cells. This procedure permits the analysis of genomic DNA methylation when only limited numbers of cells are available.