The Eaton-Littler ligament reconstruction is widely used for thumb carpometacarpal (CMC) instability, yet the existing literature lacks a thorough analysis of the outcomes for nontraumatic instability. This study aimed to assess the outcomes of the Eaton-Littler ligament reconstruction and to identify prognostic factors associated with postoperative pain. Patients with nontraumatic CMC joint instability, unresponsive to conservative treatment, were included in this prospective study. The Visual Analogue Scale (VAS, range 0-100) for pain and the Michigan Hand Outcome Questionnaire (MHQ, range 0-100) total score were measured at intake and three and 12-months postoperative. Multivariable linear regression was used to analyze the association between preoperative variables and the 12-month MHQ pain score. Seventy-four patients undergoing Eaton-Littler ligament reconstruction were included. The median VAS pain score improved significantly (P<0.001) from intake (70 [interquartile range (IQR) 63-78]) to 12 months postoperatively (27 [IQR 7-56]). The mean MHQ total score also improved significantly (P<0.001) from intake (52, standard deviation (SD) 13) to 12 months (74, SD 17). All thumbs were stable at follow-up whilst preserving the range of motion. Grip and pinch strength also improved significantly following surgery. Undergoing a concurrent surgery during ligament reconstruction and a better MHQ pain score at intake were found to be predictors of a favorable postoperative MHQ pain score. Patient- and clinician-reported outcomes improved significantly at three and 12 months after Eaton-Littler ligament reconstruction. We advise concurrent hand pathologies resulting from instability (e.g., tendinitis, synovitis) to be treated simultaneously during ligament reconstruction. Therapeutic, II.