Background:   Proteus   mirabilis   is   thought   to   contribute   in rheumatoid arthritis (RA) development in susceptible individuals through molecular mimicry mechanism. This study was detecting the prevalence of asymptomatic bacteriuria (ABU) caused by Proteus mirabilis in RA patients to shed light on its rule in RA pathogenesis.   Submit Date 2019-03-25 Revise Date 2019-04-17 Accept Date 2019-04-21     Methods: This work has been conducted in medical Microbiology and Immunology Department and Rheumatology and Rehabitilation Department, Faculty of Medicine, Zagazig University, during the period from May 2017 to May 2018 over a period of 12 months. This study is a case control included 70 RA patients (66 females and 4 males), with age ranged from 25 to 65 years, and 70 healthy controls (67 females and 3 males), with age ranged from 24 to 65 years. Two consecutive urine samples one week interval were collected from each participant for urine count and culture. Blood samples were collected from each participant for detection the level of IgG antibodies against both Proteus mirabilis and E. coli (the most frequent isolated organism from the RA urine) by homely prepared ELISA. Results: The ABU was detected in (40%) of RA patients and in (4.3%) of healthy controls. The most common isolated organisms were  E.  coli  (50%)  followed  by  Proteus  mirabilis  (25%).  A significant  difference between both studied  groups regarding  IgG antibodies levels against Proteus mirabilis was detected (P< 0.001). No significant difference was observed between both studied groups regarding   IgG   antibodies   levels   against                                E.   coli   (P=   0.902). Significant positive correlation was found between Proteus mirabilis IgG antibodies levels and the levels of ESR and CRP in RA patients. In  conclusion:  Proteus  mirabilis  seems  to  have  a  role  in  RA development.