Objectives: To identify (combinations of) prognostic indicators for the long term success of splinting in patients with electrophysiologically confirmed idiopathic carpal tunnel syndrome (CTS). Methods: This study was conducted within the framework of a randomised controlled trial on the efficacy of splinting and surgery for CTS. Patients randomised to splinting received a wrist splint, which they had to wear during the night for at least six weeks. To assess the long term success, patients were asked to indicate whether there was any improvement 12 months after randomisation. Potential prognostic indicators included variables from the history taking and physical examination, self administered questionnaires on severity of symptoms, and electrodiagnostic studies. Multiple logistic regression was used to identify (combinations of) prognostic indicators. Results: Of the 89 patients randomised to splinting, 83 attended the follow up measurement at 12 months, of whom 60 reported improvement. However, 34 patients had received one or more additional types of treatment during the follow up period and were therefore considered as treatment failures for splinting, resulting in a final success rate of 31% for splinting (26 of 83 patients). Only two prognostic indicators could be identified, namely a short duration of CTS complaints (one year or less) and a score of 6 or less for severity of paraesthesia at night at baseline. Conclusions: For patients to whom both factors applied, the predicted probability of treatment success, according to the model, was 62%. The overall percentage of patients who were correctly classified by the model was 78% (95% Cl 69% to 87%).