Purpose The purpose of this study was to evaluate the effects of epineurotomy on the post-surgical median nerve volume and clinical outcomes in carpal tunnel syndrome (CTS) patients with a prominent nerve narrowing. Methods This was a prospective, randomised, double-blind controlled trial. Patients ( n  = 50) were randomised (1:1) to open-field surgical carpal tunnel release followed by a longitudinal epineurotomy of the nerve (test), or to open-field release without epineurotomy (control). Results The nerve volume was slightly larger in the test group 90 days post-surgery (by 10.5 %, p  = 0.157) but not 180 days post-surgery. No relevant electropyhsiological or clinical difference between groups and no effect of the nerve volume was observed. The subjective pain reduction was slightly more prominent in the control group at 180 days. Larger post-surgical nerve volume was associated with lower pain, but only in the control group. Conclusions Even in selected CTS patients, longitudinal epineurotomy confers no benefit regarding the nerve volume or clinical outcomes over a simple carpal tunnel release.