From April to June,2004,an outbreak of acute upper respiratory tract infections(AURTI)occurred in the north area of Jiangsu Province,China.Twenty throat swabs were collected with 13 of them presenting an adenovirus (Ad)-like cytopathogenic effect on HEp-2.These were verified as Ad by the electron microscope,direct immunofluorescence assay and Ad primer-mediated PCR.Moreover,they were identified as adenovirus type 3(Ad3)by type-specific PCR and sequencing of the amplification products.Subsequent serologic studies were carried out to finally diagnose and document the outbreak.The neutralization test of paired serum of six in nine cases show obviously increased antibodies titers.The positive rate of IgM,IgG and recovery phase neutralization antibodies of the cases were 3.7%,44.4%and 59.5%respectively while those of the controls were 0%,8.3%and 33.3%respectively.The Pvalues of Chi-Square were 0.510,0.018 and 0.226 respectively.The concordance between IgG detected by ELISA and neutralization antibodies detected by the neutralization test was 61.4%and the Pvalue of Kappa was 0.070.By the serologic study,we can definitively diagnose that this outbreak of acute respiratory infections was caused by Adenovirus 3.