Laboratory diagnosis of respiratory viral infection has traditionally been based upon virus isolation and/or viral antigen identification. Recently, more sensitive and specific nucleic acid detection methods by reverse transcription-polymerase chain reaction (RT-PCR) have been developed, however, conventional RT-PCR can identify only a single suspected virus. To identify the causative agents which belong to Paramyxoviridae of respiratory virus infections, we have developed a single-tube multiplex RT-PCR using four primer sets which can amplify respiratory syncytial virus and parainfluenza virus type 1, 2 and 3 simultaneously. Assay sensitivity of single-tube multiplex RT-PCR allowed a detection in the range of $3{sim}500;TCID_{50}$ and there were no cross amplification among other respiratory viral agents based on the test using reference virus stocks. The single-tube multiplex RT-PCR was able to directly detect viruses in respiratory specimens, with virus being detected 11 of 80 samples as compared to 9 of 80 samples detected by indirect immunofluorescence or antigen detection following shell vial culture. This result suggests that the single-tube multiplex RT-PCR can be established as a more sensitive and rapid diagnostic application than shell vial assay for the detection of respiratory infection of Paramyxoviridae.