PURPOSE: To investigate the usefulness of postoperative follow-up ultrasonography following surgical treatment of carpal tunnel syndrome (CTS). MATERIALS and METHODS: Prospective postoperative follow-up ultrasonographic examinations were performed one, two and three months after surgery in twenty five wrists from nineteen patients who received the surgical carpal tunnel release. Symptoms were improved within one month following surgery in 21 cases (group 1). However, no relief or worsening of symptoms were noted in four patients after surgical decompression (group 2). The cross-sectional area of the median nerve was measured in three places: 1) the transverse plane at the level of the radiocarpal joint (RCJ), 2) the midlevel of the lunate and 3) the mid-level of the capitate. The ratios of preoperative and postoperative cross-sectional areas in each level were compared. RESULTS: The ratio of areas at each level showed that the cross-sectional area of the median nerve was decreased at the level of the RCJ and lunate in group 1 whereas it was either increased or not changed at the level of the RCJ and lunate in group 2. This tendency was most prominent at the mid-level of the lunate, especially on the one month follow-up examination. At the midlevel of the capitate, the ratio was increased on one month followup but returned to the original value three months after operation in both groups. However, an increase of the cross-sectional area on the one month follow-up at the capitate level was more prominent in group 1. CONCLUSION: The cross-sectional area of the median nerve after surgery showed a tendency to show a decrease at the RCJ and mid-lunate level but an increase at the midcapitate level for those who with successful decompression. This change was more prominent at the mid-lunate level and can be evaluated one month after surgery.