The SUPER-FIN study compared cast immobilization for six weeks with surgery (open reduction and internal plate fixation) in 126 adult patients with an isolated unstable unimalleolar Weber B fibula fracture where the ankle plate was congruent on initial radiographs but unstable according to the external rotation stress test. The primary endpoint was the Olerud-Molander Ankle Score (OMAS, range 0-100 points) after two years, with a non-inferiority limit of -8 points. 121 of 126 participants (96%) completed the study, the mean OMAS was 89 points in the cast group (n=62) and 87 points in the surgery group, with a difference of 1.3 points (95% CI -4.8 to 7.3). Cast immobilization met the non-inferiority criterion and there were no statistically significant differences in the secondary outcomes (function, pain, quality of life, movement, radiography). There was one case of nonunion in both groups; in the surgical group, superficial wound infection, delayed healing, nine hardware removals (including two postoperative infections) occurred. Overall, fewer treatment-related adverse events occurred with cast immobilization.