A reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay targeting the nucleocapsid phosphoprotein gene of infectious bronchitis virus (IBV) was developed. The detection limits for the IBV RT-LAMP assay were 10(1) 50% egg infection dose (EID(50)) per 50 microl of titrated viruses and no cross-reaction of IBV RT-LAMP was found when tested with other viruses including Newcastle disease virus (NDV), avian reovirus (ARV), and infectious laryngotrachietis virus (ILTV) due to their mismatch with IBV RT-LAMP primers. A total of 187 clinical tissues samples (88 blood, 62 kidney and 37 lung) were evaluated and compared to conventional RT-PCR. The sensitivity of RT-LAMP and RT-PCR assays for detecting IBV RNA in clinical specimens was 99.5% and 98.4%, respectively. These findings showed that the RT-LAMP assay has potential usefulness for rapid and sensitive diagnosis in outbreak of IBV.