There is a crucial need for simple and highly sensitive techniques to detect DNA in complicated biological samples such as serum. Here we present an ultrasensitive electrochemical DNA sensor using (i) single DNA hybridization with peptide nucleic acid (PNA), (ii) selective binding of [Ru(NH(3))(6)](3+) to hybridized DNA, (iii) fast NaBH(4) electrooxidation mediated by [Ru(NH(3))(6)](3+), and (iv) low background currents of NaBH(4) at indium-tin oxide (ITO) electrodes. The [Ru(III)(NH(3))(5)NH(2)](2+) formed from [Ru(III)(NH(3))(6)](3+) in borate buffer (pH 11.0) is readily electrooxidized to both [Ru(IV)(NH(3))(5)NH(2)](3+) and Ru complex with a higher oxidation state. In the absence of [Ru(NH(3))(6)](3+) bound to the DNA-sensing ITO electrodes, the oxidation currents of NaBH(4) are very low. However, in the presence of [Ru(NH(3))(6)](3+), the oxidation currents of NaBH(4) are highly enhanced due to electron mediation of the oxidized Ru complexes. The significant enhancement in the electrocatalytic activity of sensing electrodes after [Ru(NH(3))(6)](3+) binding facilitates to obtain high signal-to-background ratios. PNA and ethylenediamine on DNA-sensing electrodes significantly decrease [Ru(NH(3))(6)](3+) binding, also allowing for high signal-to-background ratios. The oxidation charges of NaBH(4) obtained from chronocoulometry are highly reproducible. All combined effects enable the detection of DNA with a detection limit of 1 fM in ten-fold diluted human serum. The simple and fast detection procedure and the ultrasensitivity make this approach highly promising for practical DNA detection.