Evidence from a recent cadaveric study has revealed that the thickness of the distal interosseous membrane (DIOM) of the forearm shows substantial individual variations, and a thick fibrous tissue known as the distal oblique bundle (DOB) was found in 40% specimens in the study. Another cadaveric study revealed that greater stability of the distal radioulnar joint (DRUJ) was obtained in a group with DOB following ulnar shortening than in a group without DOB. Thus, the presence of DOB influences the condition of DIOM and the ability of DIOM to provide stability to the compromised DRUJ. In the present study, we carried out a preoperative morphological evaluation of DIOM using ultrasound (US) and compared the US findings with intraoperative findings in order to investigate the efficacy and the meanings of this US evaluation. A total of 14 patients with various diseases or injuries of the distal forearm requiring corrective surgery were enrolled. The mean values of DIOM thickness as measured by US evaluation correlated well with those measured intraoperatively. The sensitivity of US evaluation in confirming presence of DOB was 80% when the presence of DOB bundle was judged by DIOM thickness. In addition, 4 of 5 patients with chronic DRUJ instability showed no DOB. This study suggested that US can be a useful tool in evaluating DIOM and for detecting the presence of DOB and may provide helpful information regarding DRUJ instability.