Objective. To test the intrareader and interreader reliability of assessment of joint space narrowing (JSN) in rheumatoid arthritis (RA) wrist and metacarpophalangeal (MCP) joints on magnetic resonance imaging (MRI) and computed tomography (CT) using the newly proposed OMERACT-RAMRIS JSN scoring method, and to compare JSN assessment on MRI, CT, and radiography. Methods. After calibration of readers, MRI and CT images of the wrist and second to fifth MCP joints from 14 patients with RA and 1 healthy control were assessed twice for JSN by 3 readers, blinded to clinical and imaging data. Radiographs were scored by the Sharp/van der Heijde method. Intraclass correlation coefficients (ICC) and smallest detectable differences (SDD) were calculated, and the performance of various simplified scores was investigated. Results. Both MRI and CT showed high intrareader (ICC > 0.95) and interreader (ICC > 0.94) reliability for total (wrist + MCP) assessment of JSN. Agreement was generally lower for MCP joints than for wrist joints, particularly for CT. Intrareader SDD for MCP/wrist/MCP + wrist were 1.2/6.1/6.4 JSN units for MRI, while 2.7/8.3/9.9 JSN units for CT. JSN on MRI and CT correlated moderately well with corresponding radiographic JSN scores (MCP 2-5: 0.49 and 0.56; wrist areas assessed by Sharp/van der Heijde: 0.80 and 0.95), and high ICC between scores on MRI and CT were demonstrated (MCP: 0.94; wrist: 0.92; MCP + wrist: 0.92). Conclusion. The OMERACT-RAMRIS MRI JSN scoring system showed high intrareader and inter-reader reliability, and high correlation with CT scores of JSN. The suggested JSN score may, after further validation in longitudinal studies, become a useful tool in RA clinical trials. (First Release Dec 1 2013; J Rheumatol 2014doi:10.3899/jrheum.131087).