Dengue fever, caused by any of four serotype of dengue virus, is epidemic in Malaysia. During an outbreak of dengue in Klang Valley from December 2004 to March 2005, 138 isolates of dengue virus were prospectively isolated in which 13 were monoclonal antibody-escape variant of dengue virus type 1 (MabEV DENV 1). These new variants could not be serotyped by either molecular method (RT-PCR) according to Lanciotti et al. (1992) or serological method (immunofluorescence assay (IFA)) by monoclonal antibody against each type of dengue virus. As these new variants were isolated mostly from foreigners working in Malaysia, hence the new variants are assumed to be either brought in by foreigners or a product of a recombination event. However only limited molecular data were available and hence further investigations are needed to enable better control and prevention of future dengue outbreaks. This study was conducted to sequence the complete genome of the MabEV DENV1 strain E152 to identify mutations that lead to amino acid substitutions and to determine the origin of this variant by phylogeny and recombination analysis. Along with this variant strain E152, full genome characterisation was also performed on two new DENV 1 isolates, B04 and K154. Genome sequence comparison and phylogeny analysis with other DENV 1 isolates from the GeneBank revealed that strain E152 was genetically closely related to two DENV 1 strains (FJ469907 and FJ469909) isolated from Singapore in 2003 and an isolate (AB178040) from Japan, in 2004. The DENV 1 K154 and the variant E512 are sub-grouped under genotype I while strain B04 belongs to genotype IV. Recombination analysis proved that strain E512 was not a recombinant product of any parental strains but a new strain of its own. IFA performed on the expressed envelope protein of DENV 1 strain E512 in baculovirus-insect cell system showed negative binding activity with the commercial MAB8701 (Chemicon Int, Temecula, CA, USA) but was IgG positive with DENV 1 infected patient’s serum. This indicates that the Mab-escape epitope may not reside within the envelope protein but possibly in other dengue viral proteins. Thus, the MabEV DENV 1 strain E152 is a newly discovered variant strain that escaped diagnostic laboratory detection and further testing involving viral typing is needed to confirm diagnosis. The dengue viral serotyping required more than one type of monoclonal antibodies with improved RT-PCR. .