The purpose of this prospective study was to determine the potential of MR imaging to depict morphologic alterations of the median nerve correlating with the stage of carpal tunnel syndrome (CTS). Eighteen wrists of normal subjects and 81 wrists of patients with CTS were examined. MR imaging was performed with proton-density- and T2-weighted spin-echo sequences. Staging of CTS was done on the basis of clinical and electrophysiological testing, including evaluation of the number of previous steroid infiltrations in conservative treatment. Median nerve flattening, cross-sectional area, and signal intensity were measured from the distal radius to the end of the carpal tunnel. Delineation and structure of the median nerve were recorded qualitatively by two experienced radiologists in consensus. Three major MR imaging criteria of early CTS were (a) isolated prestenotic and intracarpal swelling of the median nerve (P < .01), (b) the absence of significant flattening, and (c) a generalized increase in signal intensity retrograde to the distal radius (P < .01). The nerve showed sharply delineated contours and a homogeneous signal pattern. Advanced CTS was characterized by retrograde swelling of the median nerve to the distal radius (P < .01) and decreased signal intensity (P < .05). Demarcation of the nerve became poorer, and its signal pattern appeared fasciculated. After steroid infiltration, the median nerve was difficult to delineate, showed an inhomogeneous structure, and swelling was less pronounced than without steroid infiltration (P < .05). MR imaging yields typical morphologic findings that correlate with the duration and severity of median nerve compression. Hence, MR imaging allows staging of median nerve compression in CTS and thus may contribute to therapeutic decision-making.