From 2005 to 2009, the Institute for Medical Research (IMR), Kuala Lumpur, Malaysia received 488 serum and blood samples from hospitalized pa tients on the East Coast of Peninsular Malaysia, suspected of having dengue in fection. In this study we determined the prevailing dengue serotypes using a real time polymerase chain reaction assay (RT-PCR). All 4 dengue virus serotypes were found circulating during the study period; however the predominant serotype varied. In 2005 and 2006, the predominant serotypes circulating were DENV-1 and DENV-3, in 2007, DENV-1 and DENV-2 were predominant, and in 2008 and 2009, DENV-3 was the predominant serotype