〔Abstract〕 Abstract Objective To assess the epidemiological , drug resistance , and their relative risk and prevalence for yielding clinical diseases of Streptococcus mitis group (SMG) bloodstream infections in recent years . Methods A total of 50 blood culture specimens were collected from patients with SMG bloodstream infection . These SMG iso- lates were identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry ( MALDI-TOF MS) . The susceptibility to antibiotics was tested by minimal inhibitory concentrations and Kirby-Bauer (K-B) disk diffusion methods . The data were comprehensively analyzed by statistical software combined with clinical data. Re- sults Five strains were identified in SMG bloodstream infection by mass spectrometry , namely Streptococcus oralis/ mitis (S. oralis/mitis) , Streptococcus pneumonia (S. pneumonia) , Streptococcus gordonii (S. gordonii) , Streptococ– cus sanguinis (S. sanguinis) , and Streptococcus parasanguinis. These SMG showed high resistance to erythromycin and clindamycin , but low resistance to penicillin , ampicillin and ceftriaxone . Reduced hemoglobin and albumin , elevated C-reactive protein and procalcitonin were the common hematological changes in patients with SMG blood- stream infections . In SMG bacteremia , S. gordonii , S. sanguinis and S. orals/mitis were the leading group causing infective endocarditis . Patients with myocardial disease factor were more likely to cause infective endocarditis by S. gordonii and S. sanguinis , compared with S. orals/mitis. S. oralis/mitis bacteremia more occurred in patients with renal transplants progressing to pulmonary infection . Conclusion In this area , β- lactam antibiotics are the best choice for treating SMG. SMG species with closely related phylogenetically show different prevalence and risk of clinical disease in bloodstream infection patients . Early prevention and diagnosis of bacteremia caused by SMG are necessary for clinical diagnosis , treatment and effective control of infectious diseases progression .