A novel assay for the voltammetric detection of 18-bases DNA sequences relating to Chronic Myelogenous Leukemia (CML, Type b3a2) using methylene blue (MB) as the hybridization indicator was reported. DNA was covalently attached onto a glassy carbon electrode (GCE) through amines of the DNA bases using N-hydroxysulfosuccinimide (NHS) and N-(3-dimethylamion)propyl-N'-ethyl carbodiimidehydrochloride (EDC). The covalently immobilized single-stranded DNA (ssDNA) could selectively hybridize with its complementary DNA (cDNA) in solution to form double-stranded DNA (dsDNA) on the surface. A significant increase of the peak current for methylene blue upon the hybridization of immobilized ssDNA with cDNA in the solution was observed. This peak current change was used to monitor the recognition of CML DNA sequence. This electrochemical approach is sequence specific as indicated by the control experiments in which no peak current change was observed if a non-complementary DNA sequence was used. Factors, such as DNA target concentration and hybridization conditions determining the sensitivity of the electrochemical assay were investigated. Under optimal conditions, this sensor has a good calibration range between 1.25x10(-7) and 6.75x10(-7)M, with CML DNA sequence detection limit of 5.9x10(-8)M.