Objective：To investigate the clinical manifestation and laboratory examination results of infectious mononucleosis（IM） and to improve its clinical diagnosis level. Methods：Clinical data of 39 patients with IM admitted to our department from April 2011 to August 2013 were analyzed retrospectively. Results：Patients aged between 16 and 30 years old accounted for 71.8%. All patients were recovered completely except one severe case developing to Guillan Barre syndrome. Most common clinical symptoms included fever（82.1%），isthmitis（76.9%） and cervical lymphadenopathy（74.4%）. Besides，incidences of splenomegaly，rashes and hepatomegaly were 46.2%，41.0% and 2.5%，respectively. Cases complicated with Graves’ disease or other virus infections were found. Laboratory examination results showed that 66.7% IM patients had white blood cell count more than 10×109/L；87.2% had increased lympho－cyte ratio more than 35% and 72% more than 50%；84.6% had atypical lymphocyte ratio more than 10%；89.7% had liver function damages with increased alanine aminotransferase and aspartate aminotransferase levels. All patients had abnormal lactate dehydroge－nase enzymes and 69.2% had positive Epstein-Barr virus virus capsid antigen antibody（EBV-VCA-IgM antibody）. Conclusion：Chances of misdiagnosis and missed diagnosis could be decreased via better understanding of the diverse clinical features in adult patients with IM.